Found quite a good website for general snake info

http://simonsnakesite.tripod.com/snakebasics2.html

Snake History and Evolution

It is thought that snakes first appeared between 100 and 150 million years ago. No one knows exactly what the ancestors of snakes are, but it is assumed that it was some type of lizard that started to burrow into the ground, and eventually lost its legs because it didn't need them anymore. Certain species of lizards such as skinks (Scincidae), and anguids (Anguidae), have even shown a tendency of their legs to get smaller or even completely disappear. Current opinion is that monitor lizards are the ancestors of snakes. There is some evidence to support this theory; for instance, both snakes and monitor lizards have the forked tongue and Jacobsen's organ to scent their prey. Although at the present, there are no known monitor lizards without legs (if you have one, drop me a line) that doesn't mean that there never was a monitor without legs; they may have become extinct along the way.

The fossil record of snakes is incomplete; very early snakes were similar to burrowing snakes of today (wormsnakes, blindsnakes), so the bones were very delicate, making fossilization all but impossible. Snakes have gone through a long process that we only have fuzzy idea of. More "primitive" snakes, such as boas and pythons, have spurs (vestiges of legs), on each side of the cloaca; apparently males use them to aid in mating. Snakes considered most primitive include:

boas and pythons (boidae)
blind snakes (typlopidae)
threadsnakes (leptotyphidae)
animalepids (animalepididae)

Colubridae is the largest family of snakes with an estimated 1,500 species. It is sort of lumped together, though undoubtably in the future it will be broken into smaller families. Most colubrids are non-venomous and harmless, although some colubrids loosely called back-fanged snakes are venomous. They have enlarged, grooved teeth at the back of the mouth connected directly to the venom glands. Prominent examples of back-fanged snakes are: the hognose snake (I bet that was a surprise), the mangrove snake, and the boomslang (one of the few colubrids which are known to be able to kill people). Even though this group of snakes are venomous, most are not harmful because of where their fangs are located and their venom is usually weak.

Viperidae is another large family of snakes with 220 distinct species that live in both the Western and Eastern hemispheres. Most vipers are heavily built and have large heads that aid them in eating very large prey. Vipers have folding fangs that unfold when they open their mouths wide. There are two main sub-families of Vipers: Viperinae and Croatalinae. The former are the "true" vipers (such as Rhinoceros vipers and Gaboon vipers) and the latter are the "pit" vipers (such as rattlesnakes, copperheads, bushmasters). Some people classify these 2 sub-families (Viperinae and Crotalinae) as each having its own family. Rattlesnakes are considered by many to be the most advanced and recently evolved snakes, as they have adapted so that they have rattles on their tail to warn large animals that might accidentally step on them (not accidentally in certain species).