Who is considered a zonah -- forbidden to marry a Cohen? Is a girl who had premarital relations with a jew considered a zonah? If yes, what if she was raped?

In general, if a woman has sexual relations with a man she is Halachicly prohibited to marry; she becomes a Zonah and can't marry a Cohen. If she had even multiple sexual relations, but none, with which she can't halachicly marry – she is not a Zonah. Therefore for instance, if a woman had relations even once with a non-Jew, since they can't marry him, she is a Zonah; on the other hand, even a prostitute that had only Jews - is not a Zonah. For this matter even forced relations count. (See Shulchan Aruch EH 6; 8)