Otaku Dictionary: Suffixes

Suffixes
In Japanese suffixes are added to a person's name. These suffixes show the relation of the speaker to the person. Calling someone "Mary-san" is very different than calling them "Mary", "Mary-chan", or "Mary-sama". Translated manga will sometimes retain the suffixes, and one can hear the suffixes when watching non-dubbed anime.
-chan: used for feminine people or things that are either younger or of a lower position than the speaker, or very friendly with the speaker. The female version of "-kun"
-kun: used for more masculine people or things that are either younger or of a lower position than the speaker, or very friendly with the speaker. The male version of "-chan"
-sama: used for people above the speaker, the next step above "-san". Ex: "It is a pleasure to meet you, Mr.. President-sama"
-san: used for people the speaker is not very common with and on similar grounds with, such as one's classmates or coworkers (that are on the same level). The most common one, and the "safest" in speaking.
-senpai: used for an upperclassman, and someone who will never notice you. Really used for a mentor of sorts, but not a boss. Can also be used by itself without the person's name. "Thank you so much for your lessons, Senpai!"
-sensei: used for a teacher or boss. "Thank you for today's lesson, Kenjiro-sensi"
*No suffix*: This shows very strong familiarity. You would not use this on a classmate you barely speak to, but on a good friend.