Category 3 - loosely twisted pair - tested to 16 MHz (commonly used for phone wiring, also used for 10 Mbps ethernet)
Category 5 - tightly twisted pair - tested to 100-125 MHz (commonly used for 100 Mbps ethernet)
Category 5 Enhanced - similar to CAT 5, with more stringent specifications for Near End Crosstalk (NEXT)
Most CAT 5 would meet CAT 5e specifications if tested...
Category 6 - wire is a larger gauge - longitudinal separator between pairs - overall cable is thicker than CAT 5 - tested to 250 MHz (commonly used for 1000 Mbps ethernet)
Category 6 Augmented - more substantial separator between pairs - jacket is thicker to add space between adjacent cables - thicker still than CAT 6 - tested to 500 Mhz
Category 7 - doesn't officially exist yet...
See this post for pictures:
http://forums.mikeholt.com/showthread.php?t=123380&p=1170416#post1170416