hmmmm little help please

alright so im driving my car and the battery light starts blinking and then about 15 minutes later it dies..........at my house i jump it and the i let it run there for about 15 minutes so whatever it is can recharge (im not good with cars) but then instead of recharging it just dies again.......would this just be a totaly dead battery or does it sound like somthing is wrong with my alternator? thanks

blah im so stupid with this kinda stuff