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In other words, why is a boa a boa, and what makes a python a python?
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Boas give live birth and Pythons lay eggs. There are most likely some environmental differences and internal physical differences, but I am not quite sure about what those would be.

Chris
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Chris Law
U.A.P.P.E.A.L. (Uniting a Proactive Primate and Exotic Animal League)
Herpetoculture Element Representative

Another is that boas live in South and North America, while pythons are found through Africa and Asia.

Sometimes they can look surprisingly similar, like Emerald Tree Boas and Green Tree Pythons. Despite that they live so far apart from eachother. Convergent evolution, filling niches and all.
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"Another is that boas live in South and North America"

Except for old-world boas.
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>>Except for old-world boas.

Hm, not familiar with those. Which species and where are they located in the world? (in general anyway)
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Examples would be Sand boas (http://www.kingsnake.com/sandboa/sandboa.html) and Dumeril's Ground Boas (http://www.geocities.com/shavano08/dumerils.html).
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To be fair, sand boas are as closely related to pythons as they are to the "true boas."

The problem is that there are more than just boas and pythons. There are several subfamilies within the family boidae, one of which is the boinae (the boas) and another is the pythoninae (the pythons). However, there is also another family, Erycinae which includes around 15 species of snakes.

So what are the differences - traditional answers -

1. Boas = new world, Pythons = old world.
- this doesn't work because some of the boas occur in Madagascar and some of the Pacific islands. The pythons are restricted to the "Old World" if you include Australasia as part of that world. There are no pythons in the New World.

2. Boas = live birth; Pythons = eggs
This works for the subfamilies boinae and pythoninae, but some of the erycine "boas" are egg-layers, others are live bearers. Furthermore, the egg-laying erycines (some of the sandboas) lay eggs that hatch within a month of being layed as opposed to the usual 2-3 month incubation. So they are kind of halfway between the true "egg-layers" and the "live bearers".

To be honest, I don't know exactly what characters are used to unite all the pythons into the subfamily pythoninae or the boids into the subfamily boinae. The erycines all share a similar structure to the tail vertebrae.

Of course, there are other snakes that are called boas or pythons that aren't either -
Bromeliad Boas (Ungaliophis) - not boas at all.
Mexican Dwarf Python (Loxocemus bicolor) - not related to pythons
Round Island Boas (Bolyeridae) - again, not real boas
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Chris Harrison
Central Texas