The pinwheel is apple's Wait cursor, much like apple has an hourglass. The fact that everything stops means that the GUI is frozen and will not respond. As for the cause? Knowing nothing about your current situation, this can be just about anything... The problem could be as simple as a run-away application, which can be identified using Activity Monitor, or it can be as complicated as a hardware failure... For the time being, lets be positive and assume it is a Run away application... Open Activity Monitor and see if one of the applications suddenly starts using large amounts of memory right before a jam.