it's an activity or a process, BUT it happens at several levels below where you, as a user, work so you don't need to know about it. For instance opening an internet window might create a dozen different processes each with it's own thread. If a computer can only cope with one at a time then all of that dozen have to happen sequentially before you can do anything, if it can cope with 4 at a time then some, but not all, of them might happen at the same time.