AC = Alternating Current
Basically alternating current well... alternates from positive to negative. I'm not sure if this is right but it looks like a sine wave when graphed and keeps alternating over a period of time. 60hz a second from the wall and if you do the math to find the period, that would give you how many times it goes up and down per second. Whereas DC or Direct Current doesnt alternate but instead stays at one voltage the entire time. (Thats all I know?... My God I need to learn more) sorry if this didn't help. The internet would be your best bet, just type it in to any search engine