(Yes I know, I know, I will try to post more frequently.)

In Japanese conversation, the suffixes -san and -chan are used regularly when addressing other people - e.g. "Sato-san", "Kobayashi-san", "Mayumi-san", "Taro-chan"*. These are sort of the equivalant of 'Mr.' 'Mrs.' 'Ms.' etc. in English, but not exactly the same. If you have been studying Japanese for any length of time, you probably already know that there are some rules as to when to use -san or -chan, but just in case, here they are.

Use -san as the default

Whenever you are addressing someone, unless you become very close to them, you should always use -san. As a matter of fact, unless you are told otherwise by that person, you should always address them by their family (last) name + san. For example, if you met Taro Yamada**, you would call him Yamada-san unless he tells you to call him by his first name, Taro. Then, you would call him Taro-san unless you became very close friends with him.

For older people or people above you in station, use -san or their title/station

Any person older than you should always be addressed with a -san. However, if that person has a specific relationship to you, then you often use their title instead. For example, your teacher (先生 せんせい sensei) is usually addressed as [their last name]-sensei; using =san would be regarded as being disrespectful. Native-speaker language teachers often tell their students to call them by their first names, but even then the natural inclination of a Japanese person is to attach a -sensei to that, e.g. Eric (Erikku)-sensei. Writers and other people held in high regard are often called -sensei too, even if they're not teachers.

People above you in a company are usually addressed by their title only, omitting their name entirely - unless you are talking about them in the third person. For example, let's say your division head (課長 かちょう kachou) is called Hiroshi Suzuki. To his face, you would address him simply as kachou. When talking about him to someone else, you might refer to him as Suzuki-kachou.

Use -chan for children

Anyone who has not reached the age of maturity (20 in Japan) can be called -chan, usually using their first name, by any (older) adult.

Be cautious when using -chan otherwise

With the exception of addressing children, using -chan can be a bit difficult. Keep in mind that -chan is used to imply one of the following:

as a term of endearment or intimacy

to imply that the person you're addressing with -chan is somehow below you in some way

You can safely call people you are very close to (but not your elders) as -chan - your girl/boyfriend, your close friends, your younger siblings/cousins, your children/grandchildren/nieces and nephews. But -chan is also often used to address women/girls for example, up to the age of say 30 or so (the upper age limit is creeping up). Most Japanese female celebrities for example, unless they are ancient and revered, are called -chan by their fans. (Some younger male celebrities are also.) I could say a lot about the infantilization of women in Japanese society and such, but well, there it is.

When addressing a woman though, you should avoid using -chan unless you become friendly enough, if you wish to be polite. The only women who are straight out called -chan by strangers are those who are there to 'entertain' their clients (sexually or otherwise) - such as 'club hostesses', 'maids' in anime cafes, and so on.

Sometimes, a non-Japanese person will write an email or comment to me and attach -san or -chan. Maki-san is fine, but Maki-chan makes me flinch a tiny bit instinctively. No I'm not offended if you have called me Maki-chan in the past! But hey - you didn't know any better. ^_^; This is one of those subtle things that may be a bit hard to understand about Japanese societal mores, but I hope this has helped a bit.

Notes

*- Other name suffixes used are -kun and -sama. Briefly, -kun is used mainly to address boys/young men, and by superiors when addressing subordinates in company or school situations for both genders. -sama is a very formal suffix, used in well, formal situations, or in certain anime and manga etc. as a form of affectation.

**- "Taro Yamada" or 山田太郎 (yamada taro) is sort of the Japanese equivalent of John Doe in American-English - a generic male name. (The female equivalent is "Hanako Yamada" (山田花子 yamada hanako).