A few additions:
An "e" stuck on the end of the category denotes "enhanced", therefore category 5e cable is enhanced category 5 cable. This simply means that the physical and electrical characteristics surpass those of the numbered category. An "enhanced" category or a larger number improves performance due to better handling of crosstalk, attenuation, impedance, and more.

ANSI/TIA/EIA-568-A is the main standard for UTP. This standard specifies up to category 5e. Any company who claims its product is category 6 compliant is referring to a draft of a standard - and possibly not even the draft of the ANSI standard.