Frank DeFulgentis in his book 'Flux' distinguishes between a rumination, obsession, and compulsion: The rule of thumb according to the DSM is that a thought is an obsession once it becomes intrusive. In other words, if at first a concern runs through your mind it may just be a passing thing. But if it keeps coming back and it makes you feel uncomfortable, it has become intrusive and so is an obsession.
A rumination is an open ended non-solution, in other words you are not dealing with it in any specific way whatsoever (its more of a general worry, you don't want to end it or find a solution, you just keep harping on it)...you just keep being concerned with it and thinking about it over and over.
A compulsion is a specific reactive solution. In other words you touched a dirty countertop and are wondering what it was over and over....so you wash your hands. Or a covert or mental compulsion would be if you replay the event of touching the counter over and over to specifically to resolve the anxiety and put the issue to rest. You have chosen to indulge the anxiety specifically , so it is not an open-ended rambling (rumination) anymore.