Originally Posted by Lycanthrope

In a nutshell:
Category 5 - original 100 mbps standard (superceded by 5e)
Category 5e - more stringent testing than the old cat 5. Typically used in 100 mbps networks, but will support gigabit ethernet.
Category 6 - gigabit ethernet standard, will more than likely support 10 gps

RJ-45 is a pinout scheme not a type of cable :P