OCD is an illness of the brain whereby patients are tortured with repeated, intrusive thoughts and images (obsessions). These thoughts may be illogical and distressing- like violent images for a pacifist, or blasphemous thoughts for a Christian.
To settle the distress of these thoughts, there are often ritualised behaviours (compulsions).
OCD in varying forms is probably far more common than we realise. But serious OCD, that level of OCD that impairs functioning and impacts on quality of life, stands at about 2% of the world wide population.