Who is A? in pretty little liars?

it is Ali. You see: ali has a twin sister named Courtney. courtney stole ali's identity and that made ali tell her parents. they didn't believe her so they put ali (which they thought was courtney) in a mental hospital. Courtney steals Ali's life. Then ali kills courtney and she is ali the whole time. in the first one, mona is A and then ali is A
if this confuses you, go on wikipedia

A is not one person. Many people help A to ruin girls life. We'll find out who is the main A in last episode of PLL I hope.