Obsessive-Compulsive Disorder (OCD) is where a person feels compelled to repeatedly carry out a certain regime. The obsession they have is an unpleasant thought, picture in their mind, or a doubt they have. A belief that something bad will happen. The obsession causes them anxiety, this then leads to the compulsion to carry out these repeated acts. It is a repetitive cycle and symptoms can range from mild to severe depending on the individual. With some people it can take up a little of their day, with others it can practically take over their whole life.
Many people worry and are a little anxious and will for example check that the door is locked. However they are not obsessed, it is not a compulsion. Examples of compulsion can be repeatedly washing your hands in order to feel clean, obsessing about a number e.g. having to buy four of everything, or having to repeatedly check something.
The obsessions people have vary widely. They could be intrusive thoughts that they will significantly harm someone, if they are religious they may have blasphemous thoughts that go against their religion, or they may have a fear of contamination thinking they are going to get some terrible disease from touching a toilet. The repeated acts give them a short respite, a sense of release from their obsession although this is always short lived. This then causes the repeated cycle.
Obsessive-Compulsive Disorder can significantly affect the day to day running of your life, especially if your symptoms are severe.