Japanese name suffix - Generally used for men younger or who are friendly with oneself, but not always. Replaces san. Implies frienliness and/or informality - you would not refer to strangers or superiors with -kun, nor would you refer to someone you normally call kun at a formal occasion, such as an Omiai.

A: konbanwa, Bsan!
B: konbanwa, Akun! genki desu ka?
A: hai, ogenki desu.

A: ja mata ne, Bkun!
B: ja, AKun! ki o tsukete!