Whats behind a car crash test?

A crash test is a form of destructive testing usually performed in order to ensure safe design standards in Crashworthiness and crash compatibility for automobiles or related components.
Types

Frontal-Impact Tests, which is what most people initially think of when asked about a crash test. These are usually impacts upon a solid concrete wall at a specified speed, but can also be vehicle-vehicle tests. SUV's have been singled out in these tests for a while, due to the high ride-height that they often have.
Offset Tests, in which only part of the front of the car impacts with a barrier (vehicle). These are important, as impact forces (approximately) remain the same as with a frontal impact test, but a smaller fraction of the car is required to absorb all of the force. These tests are often realized by cars turning into oncoming traffic. In the U.S., this type of testing is done by the IIHS and EuroNCAP
Side-Impact Tests: these forms of accidents have a very significant likelihood of fatality, as cars don't have a significant crumple zone to absorb the impact forces before an occupant is injured.
Roll-over Tests which tests a car's ability (specifically the pillars holding the roof) to support itself in a dynamic impact. More recently dynamic rollover tests have been proposed as opposed to static crush testing (video).
Roadside hardware crash tests are used to ensure crash barriers and crash cushions will protect vehicle occupants from roadside hazards, and also to ensure that guard rails, sign posts, light poles and similar appurtenances do not pose an undue hazard to vehicle occupants.
Old versus New designs, often an old and big car against a small and new car Ford Fiesta Vs Ford Sierra,Renault Modus Vs Volvo 940, or two different generations of the same car model. These tests are performed to show the advancements in crashworthiness