Venom is modified saliva. It's primary function is to capture/kill the prey and then it also helps to digest the prey.
Some venoms are referred to as hematoxic which means that they primarily affect the blood. Almost all american pitvipers fall into this category. A hematoxic venom destroys tissue and is very painful.
Neurotoxic venoms attack the nervous system and brain. These may cause almost no pain, but shut down the respiratory systems and interfere with heart functions. Good examples of these types of snakes are the cobra and coral snakes
Snake venom is made up of about 20 different enzymes. Species usually have 6-12 of these enzymes. These enzymes determine the toxicity of the snake and whether it is hemotoxic or neurotoxic.