OCD is Obsessive Compulsive Disorder, an anxiety or mood disorder. Sufferers of OCD have intrusive thoughts that lead to fear, worry or anxiety. These thoughts are persistent, in some cases leading to an intense state of panic. This is the "obsessive" part. To alleviate this fear and anxiety, the sufferer will carry out an action (compulsion) often repeatedly until the anxiety and fear goes away.
What's important to note is that OCD sufferers KNOW their actions are irrational and unnecessary, but they feel compelled by fear to do them anyway. This adds to the aggravation and humiliation some sufferers feel.
A common example of OCD is excessive hand washing. The person fears germs and believes there are many germs on their hands. This leads them to scrub their hands until they feel they are clean. For some people this is a set time (like 5 minutes) while for others it is a systematic ritual of turning the hands a certain way under the water. Some people have to space their footsteps a certain distance apart, others have to walk in and out of a doorway several times, some check over and over to see if their car is locked.
Germaphobia and hand washing are by no means the only manifestation of OCD. There are checking, arranging, cleaning, counting and superstitious rituals to name a few. Everybody does these actions now and then, but if these actions are brought about by an intense fear, anxiety, guilt, or dread, cause you distress, and get in the way of your life - talk to your doctor.
OCD and other mental illnesses are nothing to be ashamed of. If you or someone you know suffers, it's important to get help.
All the best : )