Compulsions are repetitive behaviours or mental acts that a person feels compelled to perform in response to their obsessions. These behaviours are often performed in an attempt to reduce anxiety or prevent something bad from happening. Common compulsions include excessive handwashing, checking locks or appliances repeatedly, counting, and arranging items in a specific order to name just a few.
Compulsions are also known as safety or neutralising behaviours.
There are two main types of compulsions. The first are checking or verifying behaviours. They tend to physical or mental. The main purpose of these is to feel totally sure it is okay and/or put it right if its not. The second, we call, restitution. This is a fancy CBT word for correcting something that has they believe has already happened. An simple example to make this more understandable is cleaning something that a person believes has become contaminated or thinking a positive thought to cancel out a negative thought that they think might be dangerous.
The two most important reasons for compulsions is to try to prevent harm you might feel responsible for actually happening and second to reduce anxiety or feeling uncomfortable (that could be anxiety but also shame, guilt etc)