OCD is a mental illness in which one must obsess over every or most things done in life that are simple with others who don't have the illness (I.e. having to count pennies a certain amount of times or they feel bad about not counting them)
Makes life go slower for a) you and b) everyone around you
Can be dangerous in a life-or-death situation
drives both a) the person who has it and b) the people around them crazy
the pros: (my mom says this)
you can focus for hours and hours on something you really like (like math homework? It could be easier because you obsess over it and strive to get good at it)
a very few more
Check out Wikipedia for more info!