Car crashes fall into several major categories (whose names are self-explanatory): 

Head-on collisions

Rear-end collisions

Typical scenarios for rear-ends are i) a sudden deceleration by the first car (for example, to avoid someone crossing the street), ii) the following car that does not have the time to brake and impacts the first and iii) at a road junction the following car accelerates more rapidly than the leading.

As a rule of thumb, impacting into another car is equivalent to impacting into a rigid surface (like a wall) at half of the speed. This means that rear-ending a still car while going at 30 mph is equivalent, in terms of forces, to impacting a wall at 15 mph. The same is true for the impacted vehicle.

For purposes of insurance and policing, the driver of the car that rear-ends the other car is almost always considered to be at fault due to not being within stopping distance or lack of attention. An exception to this rule comes into play if the impacted vehicle is in reverse gear.

Side collisions

Broadside or T-bone collision

Broadside collisions, commonly known as T-bone collisions, are where the side of one vehicle is impacted the front or rear of another vehicle or a fixed object. Vehicle damage and occupant injury are more likely to be severe, but severity varies based on the part of the vehicle that is struck, safety features present, the speeds of both vehicles, and vehicle weight and construction.

An occupant on the struck side of a vehicle may sustain far more severe injuries than an otherwise similar front or rear collision crash.

Side-impact airbags can protect vehicle occupants during side collisions, but they face the same limitations as other airbags. Additionally, side impact wrecks are more likely to involve multiple individual collisions or sudden speed changes before motion ceases. Since the airbag can only provide protection during the first collision, it may leave occupants unprotected during subsequent collisions in the crash. However, the first collision in a crash typically has the most severe forces, so an effective airbag provides maximum benefit during the most severe portion of a crash.

Sideswipe collisions

Sideswipe collisions are where the sides of two parallel vehicles touch. If the vehicles are traveling in the same direction and neither vehicle loses control, the collision energy is minimal and the vehicles may suffer only cosmetic damage. However, loss of control of either vehicle can have unpredictable effects and dramatically increases the expected crash severity. Sideswipe collisions are frequently caused by a failure to control a vehicle. 

Multi-vehicle collisions

A multi-vehicle collision (colloquially known as a pile-up) is a road traffic accident involving many vehicles. Generally occurring on highways, they are one of the deadliest form of traffic accidents.

They are particularly deadly as the solid mass of crumpled vehicles makes escape difficult. A fire in one part of the accident can quickly spread to spilled fuel and cover the entire crash area. Vehicles in a pile up are often hit multiple times, increasing risk of injury to the passenger. Moreover cars are often spun during an accident and are subsequently hit from the side, increasing risk of injury or death. Some vehicle occupants choose to get out of their vehicles during a pile up, making them vulnerable to oncoming vehicles. Pile-ups can also overburden local firefighting ambulance, and police services making quick rescues more difficult.

Determining the cause of such accidents is also difficult for investigators and it is often impossible to tell if negligence caused the crash.