I know "-chan" among girls is used in modern times for familiarity and basically in reference to cuteness.. "-kun" is used for boys and subordinates in work situations(Teachers use "-kun" to refer to students I think), um... "-sempai" is for students older then oneself, "-kouhai" is the opposite... there's more to it then that but that's in basic terms.

... And now I'm surprised we don't have a girl who refers to one of the others as "onee-sama!"