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A 'venomous snake' is a snake that uses modified saliva, venom, delivered through fangs in its mouth, to immobilize or kill its prey. (In contrast, most non-venomous species are constrictors which suffocate their prey.) Venomous snakes include several families of snakes and is not a formal classification group used in taxonomy.
Venomous snakes are commonly called poisonous snakes. But some people in talking about biologically produced hazardous substances restrict the word "poisonous" to organisms which make a poison which is not a venom and which does not have to be injected to take effect.
Venom is different from other poisons. Other poisons affect the body by entering the digestive system, while venom affects the body by getting into the blood stream and tissue. Drinking snake venom is harmless as long as the venom does not enter the blood stream, e.g. through a laceration inside the digestive tract.^[1]
Boids (boas, pythons, etc.) are not venomous, but their bites should be attended to medically. Their teeth are long and sharp, capable of inflicting lacerations, with bites often introducing mouth bacteria and shed teeth into the wound.

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Families of venomous snakes

Cited references

Families of venomous snakes

Over 600 species are known to be venomous -- about a quarter of all snake species. The following groups of snakes can be aggressive and inflict dangerous, even potentially lethal bites:

Family	Description
Atractaspididae (atractaspidids)	Burrowing asps, mole vipers, stilleto snakes.
Colubridae (colubrids)	Most are harmless, but others have toxic saliva and at least five species, including the boomslang (''Dispholidus typus''), have caused human fatalities.
Elapidae (elapids)	Cobras, coral snakes, kraits, mambas, sea snakes, sea kraits and Australian elapids.
Viperidae (viperids)	True vipers and pit vipers, including rattlesnakes.

Cited references

1. Klauber LM. 1997. Rattlesnakes: Their Habitats, Life Histories, and Influence on Mankind. Second Edition. First published in 1956, 1972. University of California Press, Berkeley. ISBN 0-520-21056-5.