Kun (君【くん】?) is used by persons of senior status in addressing or referring to those of junior status, or by anyone when addressing or referring to male children or male teenagers. It can also be used by females when addressing a male that they are emotionally attached to or have known for a long period of time. Although kun is generally used for boys, that is not a hard rule. For example, in business settings, young female employees may also be addressed as kun by older males of senior status. It can also be used by male teachers addressing their female students.
BTW, -kun is a Japanese honorific. It's in Wikipedia. ;)