What OCD is
OCD is a cycle of intrusive thoughts, images or urges (obsessions) and the things you do to get relief from them (compulsions). Compulsions can be visible, like checking or cleaning, or entirely mental, like reviewing, praying, seeking reassurance or trying to cancel a thought. The relief is short-lived, which keeps the cycle going.
It often shows up in forms people do not recognize as OCD: fears of harming someone, relationship doubts, moral or religious scrupulosity, health fears, or "Pure O," where the compulsions happen in your head.