After 4. Nc3, the move is 4... Nd4. After 4. Bc6 dc 5. Ne5 Qd4! and black annexes the knight, e4 pawn, or the king (mate threat on f2.)
ruy lopez: classical defence.

Play through some games.
http://www.chessgames.com/perl/chessopening?eco=c64
4 Bc6 dxc6 5 Ne5 Bf2 6 Kf2 Qd4=
4 O-O you can play Nge7, Qf6, d6, or Nd4 (main line)
4 c3
4 ...Qf6
4 ... Nge7
4 ... Nf6
4 ... f5 the Schliemann or Jaenisch variation is the main line. A good book, if you can find it, is Spanish without...a6 by Mikhail Yudovich, The Macmillian Chess Library, 1986
Check out the videos http://www.youtube.com/watch?v=M1qgxLYZGKA

pvmike that was not the line i was looking for. i said after Nf6? same to you dmaster. If someone plays or used to play this line would post that would be nice. thanks

In that case, you could play Bxf2+ followed by Qd5 which attacks the Knight on e5 and defends everything which is completely winning for Black.

The articles here might be helpful
http://correspondencechess.com/campbell/hard/h990517.htm
These are classical lines with Nf6, there are some quite crazy lines without Nf6 too.
I was looking for some help this opening. I am playing the black side of the ruy lopez classical defence. I get confused because there are 3 variations, 1) is 4.c3, 2) is 4.Nc3, and 3) is 4.0-0. After 4.c3 The book say Nf6 is the move, but what is stopping Bxc6 winning the e5 pawn?
Next what does black play after 4.Nc3? It says it is not one of the main variations but someone played it against me when i was at the chess club tonight and i did not know what to do?