If you jump it, and it continues to run, then the alternator is still outputting. If you get it going for a bit, then it dies and needs another jump to get it going again. The the alternator is not maintaining the charge. Now, they can also become fickle at the end, working at time and not at others, that is when it can get a bit tricky. Of course, there are all sorts of lovely testing devices out there now that one can actually use to test.