Cobras have several methods for delivering their deadly venom to their prey. Some cobras can spit their venom into a victim's eyes, causing extreme pain and blindness. However, the most common and well known method of venom delivery is injection into a victim's body through their bite.
Cobras belong to the sub-group of snakes known as elapids; there are over 270 species of cobras and their relatives. An elapid's venom contains postsynaptic neurotoxins that spread rapidly in its victim's bloodstream, causing respiratory failure and, eventually, death.
Cobra venom is an example of a molecule that prohibits the interaction of acetylcholine molecules (transmitted from nerve endings surrounding the diaphragm muscle) with the receptor sites on the diaphragm muscle. (See the section on Human Respiration for more details). It binds to the receptor sites, blocking them from interacting with acetylcholine molecules. Even worse, the venom molecule will not immediately break down and vacate the receptor site, effectively removing the site from active duty.
It has been determined that even if only a third of the receptor sites on your diaphragm become blocked by venom, you will cease breathing. With cobra venoms, this process can take as little as 30 minutes. The only way to counteract the effects of cobra venom (or most other poisonous snake venoms) is to inject the appropriate antivenom shortly after the bite occurs. If antivenom is unavailable, your life can still be saved by putting you on an artificial respirator until the paralysis of the diaphragm muscle wears off.
(If this watered-down explanation of effects of cobra venom wasn't enough for you, then check out a more-detailed explanation of the effects of cobra venom.)