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.
Examples:
A:
konbanwa, B
san!
B:
konbanwa, A
kun!
genki desu ka?
A:
hai,
ogenki desu.
(later)
A:
ja mata ne, B
kun!
B:
ja, A
Kun!
ki o
tsukete!