The common Fast Ethernet medium is 100BaseTX. It utilizes two pairs of Cat5 cabling and is usually terminated with an RJ-45 modular plug. The two pairs used are pins 1 and 2, and pins 3 and 6, where the numbering starts from the left when you're looking at the tip of the plug with the clip pointed up. Each pair must be twisted together. Most installations I've seen use the orange and green pairs, respectively. The other two pairs don't have to be hooked up at all, but if they are, brown is pins 7 and 8, and blue is pins 4 and 5, but the blue polarity is reversed. So a finished cable looks like:

Orange	Orange	Green	Blue	Blue	Green	Brown	Brown
White		White		White		White

When making a crossover cable, pairs one and two must be swapped, but polarity kept, on one end. I"m not sure if pairs 3 and 4 are supposed to be swapped technically, but with 100BaseTX, it doesn't make much difference, and this type of cable is hardly ever a long run, so fixing it if 4 pairs becomes required in the future is a trivial undertaking. So:

Green	Green	Orange	Blue	Blue	Orange	Brown	Brown
White		White		White		White

The main difference between category 3 and category 5 cables is the number of twists in the pairs per unit length. This translates to being able to transmit further. You can get away with using Cat3 cabling with 100BaseTX if your run is fairly short. This is easier to do with switches because they also function as repeaters, amplifying the signal, so runs can be measured from switch to switch and switch to host, as opposed to hubs, which have no amplification feature, and, as such, runs must be measured from host to host, which increases the number of different run lengths geometrically, making troubleshooting and solving individual host to host problems much more difficult.

There is also a specification for 100BaseT4, which is designed to use all four pairs on Cat3 cabling. I've seen products advertised occasionally, but seldom. Cisco switches do have modules for them, though, I believe. There's also 100BaseFX, which is run over multimode fiber.

I hope that this puts to rest all Ethernet talk. And, please, no one bring up 10Broad36.