Everyone knows -san. In Japanese, you must attach a suffix to someone's name unless you're very close friends, and san is the most basic. You can forget trying to translate it. Some people claim it's like calling them "Mr." or "Mrs." so-and-so, but there are other suffixes which are better suited for that, and san is less a term of a respect than obligatory.

San is completely neutral, straight and level on the politeness scale. In real life, easily 75% of all the suffixes ever used are going to be san. Nextdoor neighbors are san to each other, fellow employees are san, friends you've made as adults (that you haven't grown up with) are also going to be san. Anime fans love to drop san in favor of kun or chan as soon as they learn them -- this doesn't work in Japan. You're going to either sound feminine, or insulting. Stick with san unless a Japanese person basically shows you that it's okay to use something else.

"But I'm an American! I don't like formality. I like to drop to the informal as soon as possible." The average Japanese person simply doesn't care. You are making a statement about what you think about them. The important thing is in how they interpret it. So please don't try and make a statement about your own personality by not using san when you need it - it will backfire.

　

Chan. Hideously overrused by American fanboys. Little kids and babies in Japan have a hard time making proper S sounds, and will often slur them. For example, sakana (fish) becomes chakana, sensei becomes shenshei, etc.

This is where chan came from, as a baby pronunciation of san. Keep in mind how diminuitive this is when you use it. It has a strong aura of "cuteness" and femininity to it. You can use it both for little boys and girls up until about the age of 8 or 9. After that, you should stick to only using it with cute little girls.

Yes, a number of guys use chan for themselves. I promise you that as a foreigner, you can't get away with this. Japanese males using chan have other ways within the language of establishing that they are male. If your Japanese is still fairly gender neutral textbook Japanese, and you use chan, many people will consider you comically swishy.

Any adolescent boy or older is going to get very upset if you use chan with their names, (unless you happen to be an attractive woman using it in a playful flirtatious way.) Girls use chan with one another constantly, even into adulthood, but boys tend to drop it at an early age. Don't use chan unless you're very close to the person and have heard them use it before, or unless you're an adult trying to talk to a kid, like a teacher to their 1st or 2nd grade student. Always error on the side of caution before pushing chan on someone who may not appreciate the connotation. Yes, it is used all the time in anime. Anime is not real life. Please don't over-use it.

　

Kun is mis-used almost as badly as chan. Most websites simply say that kun is the boy's version of chan. Aaaalmost, but not quite.

Kun is usually attached to boys' names, however next exclusively. It implies one of two things: 1) the person is a male who you consider yourself very close to. Or 2), They are significantly below you in social standing. Note the last one. Using kun on someone who does not expect it will be interpreted this way. Be careful.

Many American fanboys like to attach kun even to the names of people they don't know. "Look how casual and friendly I am!" No. Do not do this. If you attach kun to a stranger's name, they are going to interpret it as in #2 above, and take it as an insult. The rules are slackening, and it's becoming more widely accepted to use kun even among regular guy friends that you haven't known since childhood, but it's always safer to not use it until you receive some signal that it's okay.

Sensei is easy. Historically, sensei was used only to refer to medical doctors and professors. These days, sensei can be used with anything who teaches anything. I've heard professional athletes call their coaches sensei, manga artists call their role model artists sensei. I've even read interviews where fashion models will call their older, more experienced model friends sensei.

　

Dono is an odd one, and not heard too often. Dono is an archaic, highly formal way of addressing someone, similar to the feeling of "sir," or "m'lady," or what have you. Respectful, polite, but obscolete.

You will still see dono used in official documents, especially if they are religious in nature or highly formalized. Otherwise, it's mostly disappeared.

　

People often say that san is like "Mr." or "Mrs.", but shi comes a lot closer to this feeling. Shi is used almost exclusively in business contracts, newspaper articles, etc. It is 3rd person. This means you can't attach it to the name of someone you're talking to directly. In that case, it would be best to use san or sama. However, it's still important to know if you're ever reading something in Japanese that is more formal than manga or intro level textbooks.

　

Kyou. This is an official title similar to Sir or Lord. It is seldom used now that Japan has become a full democracy, however up until the Meiji Restoration (1870ish), it was used for government leaders, ministers, and advisers.

These days, it is rarely if ever used for anyone. Foreign dignitaries will usually be referred to with their title, not with kyou. However you will still see it used in older settings to refer to people like government officials.

Senpai (it's also sometimes spelled sempai in English, although it's technically an N) means someone who "came before" and kouhai means someone who "came after." In any organization, whether it's the military, a karate club, or a basketball team, there will be people who joined before you, and people who joined after you. Senpai is often used as a name suffix. Kouhai is not, but the two words form a pair so I included both.

In Japan, this is a big deal. Senpai are to be treated with respect. In return, they traditionally treat their kouhai with care and look out for them. In Japanese society, this fosters an extreme sense of belonging, where everyone has their own set of senpai and kouhai, and in turn they form a chain of friendship that keeps the group together. Traditionally, even if it was something as trivial as, say, the school's Aikido team, senpai watch out for their kouhai, providing them with help even outside of the club, and kouhai treat their senpai with mutual respect. Ideally, everyone comes away from the experience with a sense of worth, obligation, and friendship, however in any society you are going to have the occasional person who abuses the system. Senpai mistreating their kouhai always makes big news outside of Japan, and contributes to the stereotype of Japan being a harsh, rigid society. In actuality, this is very rare.

For better or for worse, this is slowly fading in Japan. High school and college-level athletic teams still maintain the strong senpai/kouhai relationship. Martial arts dojos are probably the strongest remaining places where senpai/kouhai is used outside of the workplace. Otherwise, clubs and organizations are becoming more lax, and more individualistic like their Western equivalents.

The point is:

Name suffixes make statements about senpai/kouhai relationship. Using a diminuitive suffix that the other person is not expecting will make it sound like you consider yourself the senpai of the relationship. This can be disastrous if the other person does not consider themselves your kouhai. This is why you must be careful.

Niisan is used to refer to young men. It technically means someone's older brother. There's no hard, official age limit on who you can call niisan, but it's basically between adolescence and middle age. If you're a guy within this age group, don't be suprised if a little kid calls you niisan or niichan. They're not trying to say you're their older brother. It's just a way of referring to young men within that age group, especially if you don't know their name. It's not even considered rude. It's as neutral as calling someone "young man."

　

Neesan is the female equivalent of niisan. It technically means older sister.

　

Obasan technically means "aunt" and is used to refer to people who are middle aged, but not yet elderly. Note that long ago, age was a good thing in Japan, and people were proud to be called obasan instead of neesan. In recent years, however, this has changed. People will be irritated if you use obasan before they're well into middle-age. People like to hang on to neesan as long as possible.

Note that the o in front of this one is mandatory. The pronunciation is off without it. There are kanji for this, but they are rarely used.

　

Ojisan (uncle) is the male equivalent of obasan.

Everything that applies to obasan applies to ojisan. Some people might not be terribly thrilled about being refered to as ojisan before they're ready to admit it. Note that the o in front of this one is also mandatory. Pronunciation just doesn't work without it. There are kanji for it, but they are rarely used.

　

Baasan means grandmother, and refers to someone past middle age. It is very common to hear baachan instead of baasan, because of the image of a cute, doddering woman in her twilight years. Again, age has historically been something to be proud of in Japan, so this isn't considered rude. These days though, calling a woman baasan before she is actually elderly is going to really irritate her.

Babaa is the rude way to refer to an old woman. Nasty old ladies, witches, etc, will get people referring to them as baabaa.

　

Jiisan means grandfather, and is the male equivalent of baasan. Again, it's also changed to jiichan to refer to little, cute, doddering old men, but I guarantee you'll find fewer elderly men who appreciate this image than women.

Jijii is the rude way to refer to an old man. Nasty old men who grope women on trains, and sit on park benches and yell at random passerbys are likely going to be referred to as jijii.

Shounen is written with the two kanji for "little" and "years." Essentially this means a young boy between the ages of 10 to 20 or so. What ages constitute shounen change depending on which person you ask. Shounen is different from niichan because it isn't a title. It's a noun. You can't stick it on the ends of names. (Yoshi-niisan works, Yoshi-shounen is comically wrong.) It's used exclusively to refer to a 3rd person within that age group, or as a reference to a target age group, such as the magazine Shounen Jump. Here it's showing the target age group for the magazine.

　

Shoujo is written with the two kanji for "little" and "girl." Why do boys get "little" and "years" but girls have to use "little" and "girl?" The best guess is that shoujo came along as a word after shounen was already in wide circulation to refer exclusively to boys, so they had to use "girl" to show the difference.

Again, this is a noun, not a title, just like shounen.

　

Seinen requires a bit of explaining. The two kanji are "blue" and "years." The color blue in historical Japanese is very strongly tied to sex, life, and reproduction in a very vague, we're-sort-of-saying-it-but-not-quite, nudge-nudge wink-wink say-no-more, sort of way. I dare you to count how many songs use the color blue in a suggestive way.

Seinen therefore refers to a young man, slightly older than shounen, who's around the age where historically he would be working on the whole "blue" thing. This is pre middle-age, but post adolescence.

No, this is not a risque term. It's completely acceptable to use in all situations.