What is the actual difference between Cat-5 and Cat-5e?

I believe that the difference between 5 and 5e is a more specific description of the twists per pair and twists per foot of each pair.

Cat5a and Cat6 further enforce the geometry of the pairs relative to each-other over the course of the entire cable and restrict the untwisted amount at the ends.

In practice, if it says Cat5, it is an old cable. "Cat5" hasn't been made in years, and any twisted pair cable suitable for use in networking (ie not cat3 or something used for a thermostat or lawn sprinklers) will be a Cat5e or better.

A true Cat5 installation is sufficient for gigabit ethernet. I've seen plenty of things labeled cat5 that are not actually up to spec for one reason or another, that run 100mb fine but don't work with gig.

You got part of it right. I won’t go into the exact standards for each, but in general:

Cat 5 is rated to 100 MHz, but was specified when only 10BASE-T and 100BASE-TX Ethernet was common. Both of those only transmit on one pair in each direction.

Cat 5e is also rated to 100 MHz, but includes specifications for testing parameters important to 1000BASE-T (Gigabit Ethernet). Gig Ethernet transmits four pairs simultaneously in parallel. If a signal leaks from one pair to another it disrupts the other pair. Thus, the new specification with extra required tests at the “far end.” [Note: 1000BASE-TX is not an Ethernet standard - more marketing and such.]

Cat 5E (capital letter) is a marketing spin produced by cable manufactures to tempt you into buying their brand instead of the cheaper alternatives. Does not apply to networking standards.

Cat 6 – hmm. Around 1998 the cable manufacturers needed something new so they proposed 200 MHz to the standards bodies. The standards bodies asked for 250 MHz instead, estimating the requirement for 10 Gig Ethernet. It is rated to 250 MHz, but no standard pushed this.

Cat 6e or Cat 6E – another marketing spin trying to capture market share before the next standard came out. They anticipated the “e”, so when the standard finally came out the standards bodies couldn’t use that letter designation.

Cat 6a – is rated to 500 MHz. When the 10GBASE-T (10 Gigabit Ethernet) standard finally came out they had additional test requirements beyond what Cat 6 was specified to deliver, so this standard was published.

Cat 7 – only exists in cabling components (cable and connectors), but not an assembled link. The closest to a next-higher cabling standard is the ISO Class F. No existing Ethernet standard requires it.

You also mentioned T568A and T568B. Electrically they are identical. However, when that was specified there were some … political differences … between various big companies on the standards committee. One camp had been using the first color code, the other camp had been using the second. The compromise was to have two color codes in the standard. As long as you only use one color code around your building you should be fine. If you mix them then you get crossed pairs.

These and lots of other exotic network trivia are found in a recently published book: Network Maintenance and Troubleshooting Guide, 2nd Edition. http://www.amazon.com/Network-Maintenance-Troubleshooting-Guide-Solutions/dp/0321647416/ref=ntt_at_ep_dpi_1