Join Date: Mar-2003
Location: Orillia, ON
When it comes to taxonomy, you have to forget about tree dwellers vs. ground dwellers, rodent eaters or not, etc. Species, genera, families, etc. have usually been differentiated on the basis of morphology (body features, such as presence of labial pits for the pit vipers). Much of early taxonomy (and lots of todays) was done in museums, where all that they had to work with were preserved specimens. They couldn't take into account behaviour, environment, etc. More recently, a lot of interesting work has been done through genetics, which has shown that some things we thought were closely related due to similar body structures are not, and the opposite is also true.
So, things that can separate taxonomic groups may not even be visible to you- it could be genetic, or it could be something like the number of vertebrae! However, when you are in the middle and upper levels of taxonomy (say Family up to Kingdom) most differences are relatively major and noticeable.
To expand on what Ryan was saying,
Family Boidae (note: some authors break this into Boidae and Pythonidae, others consider these two to be subfamilies): thought be to evolutionarily primitive snakes (not necessarily old!) with pelvic girdles and spurs, left and right lungs (though not all of them have both functioning), according to a text book I dug out, "premaxilla-maxilla articulation, premaxillary, maxillary, and palatine teeth, a coronoid element in the mandible, hypophyses on anterior trunk vertebrae, and ovoviviparity." Translation- jaw function, particular teeth, backbone shape, and live bearing. I definitely wouldn't have remembered all those! Realize that there are often exceptions to the rules of taxonomy- things that fit neatly into one category except for one characteristic which clearly doesn't. Typcially boids are found in the 'new world', North & South America.
Pythonidae: Typically found in the 'old world', Africa, Asia, and Australia. Similar to the boids, but different in skull structure, and they have paired subcaudal scales (yes, scale counts are important). All are oviparous (egg laying).
Viperidae: Venomous with hinged front fangs, plus a lot of other teeth and jaw differences.
Elapidae: Venomous fixed front fangs, plus teeth and jaw differences to go with them.
Colubridae: Snakes that don't fit into any of the other families. Some may be venomous but don't have front fangs like the others. Where present 'fangs' of any kind are grooved instead of being hollow. This is by far the largest family of snakes, and is considered to be of recent origin in the evolutionary sense. It is still an 'old' enough group to have fossils.
There are also about 10 small, obscure families, such as Atractaspididae, with only one genus, the burrowing asps, and Xenopeltidae- the sunbeam snake, which is sometimes seen in the pet trade.
I hope this clarifies things! Taxonomy is not always easy to understand, nor to keep up with, since it is always changing as new techniques are applied and new discoveries are made.