There are two groups of rattlesnakes: the genera Crotalus and Sistrurus. All have a venomous bite, and some are much more dangerous than others. Pit vipers have a set of heat-sensing "pits" on their heads which help them find prey and move towards it, even in the dark.
The name rattlesnake is used because they have a special tail with several parts on the end that can make a loud bzzzz sound, or rattle. This defends them against predators and large animals. It acts as a warning signal.