OCD is a kind of anxiety disorder that is characterized by obsessions and compulsions.
Obsessions are irrational, intrusive thoughts in your brain. They pop into your head at any time and create havoc on your day. These thoughts are disturbing and all-consuming. They loop through your mind, playing non-stop, letting you think that you are in trouble or some kind of danger. The OCD brain tricks the mind into believing that these irrational thoughts are real threats that could harm you.
For example, a person with OCD may think that their hands are contaminated and that the germs on their hands could give them a deadly disease. Or they may worry about leaving on a gas stove and then getting poisoned by the gas that is leaking from the stove.
Most of these obsessions lead them to compulsions. In order to calm their obsessions, a person with OCD feels the need to compulse. A person with OCD will complete a ritual or action over and over, in order to release their anxiety. However, the anxiety relief usually only lasts momentarily. Without completing a compulsion, that person may believe that some kind of harm could happen to themselves or their friends and family. This is why they feel such a strong need to complete these compulsions over and over.
An example of a compulsion could be washing hands multiple times, when only washing them one time would have sufficed. Another compulsion could be checking the knobs on a gas stove over and over to make sure that they are turned off.
Most of these obsessions and compulsions distract you from your day-to-day life and are frustrating, and overwhelming.
OCD is a disease of doubting yourself. That is a lot of what it is for me. Did I turn off the stove? Will the house catch on fire if I left the lights on? Did I cook the steak enough? If I eat the steak and it's not cooked all the way, will I get salmonella and die?
One other important thing to note about OCD is that no two people with OCD have the same obsessions and compulsions. As a group of people with the same disease, the root problem is the same; however, what worries me may not worry the next person with OCD. And my compulsions may not be the same as another person with OCD. We may have some similarities, but our OCD will have different characteristics and ticks. This is important so that if you're talking to someone with OCD, you don't assume that we all obsess about the same things. It's just good to be sensitive to this fact.
Generalized Anxiety Disorder (GAD) is a disorder characterized by chronic worry and nervousness. It can keep someone from performing everyday tasks. With GAD, the fear and worry a person experiences is not connected to a specific item or memory, it is a more generalized everyday anxiety or fear or dread that follow you each day. Your common everyday worries (finances, job, being on time, etc.) can explode into worries that carry over for days and keep you from living your life normally that can become blown out of proportion.
For example, "He is late! He should have been here 20 minutes ago! He probably was in a car accident!" (I am guilty of this one.)
To read more about OCD and Anxiety, you can check out these sites:
Anxiety and Depression Organization of America
Facing a Daughter's OCD
International OCD Foundation
This is Anxiety--The Atlantic Article
Bring Change 2 Mind