Broadly speaking, people who struggle with OCD experience some combination of obsessions and compulsions.
Are unwanted thoughts, images, or urges that are experienced as extremely fear or anxiety-provoking. Obsessions tend to be more intense than basic worry, and often feel very intrusive. People with OCD typically struggle with 1 or more obsessive themes. Common themes include germs/contamination, fears of hurting self or others, or religious worries. Sometimes the exact fear may be vague or ill-defined. Because obsessions are experienced as so unpleasant and unwanted, they trigger urges to do something about them.
Are mental or physical acts focused on trying to manage obsessions and the anxiety associated with obsessions. Common physical compulsions include checking the locks on doors and windows repeatedly, trying to stay scrupulously clean (e.g., excessive hand washing), or repeating body motions or movement. Compulsions may also be mental, which include counting, praying, or some other specific way of mentally trying to get rid of obsessions. People may spend several hours each day engaged in compulsions, and the amount of time they spend or the complexity of the ritual often increases over time.