If I had to explain obsessive compulsive disorder (OCD) in a few words, it would be: a disorder that causes recurrent thoughts and urges that cause the person anxiety or stress. The person feels that they must perform that urge in order to feel better.
Common urges (also called compulsions) are things like checking locks, washing hands, praying, counting. Common thoughts (also called obsessions) are things like thinking recurrently about germs, being hurt, or hurting someone.
Many times, people with OCD avoid situations that would make their compulsions or obsessions worse (like avoiding places where they may have to touch things and contaminate their hands).
Though OCD usually starts in the late teen years, it can start much earlier than that. Many children and teens with OCD also have other mental health issues and are at higher risk of suicide.
If you or someone you know are having these symptoms, please seek help from a psychiatrist. Many people with OCD do get much better with professional help.