In the Ruy Lopez, does White play on the queenside or King side? I always play on the kingside, but I recently heard from Expert class that White plays on the queenside. I also seen another Expert class play on the queenside. If this is true, then why does White play on the queenside? Also, when does White play on the Kingside?
In most variations of the Ruy Lopez, White, with the Bishop on b3, has better chances playing on the kingside.  In the Marshall gambit, White counterattacks on the Queenside.  In the Exchange variation, White normally has better chances in the kingside in the endgame.  

Fischer-Spassky
Ninth Match Game
Sveti Stefan 1992


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1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Bxc6 dxc6 5. O-O f6 6. d4 exd4 7. Nd4 c5 8. Nb3 Qd1 9. Rd1 Bg4 10. f3 Be6 11. Nc3 Bd6 12. Be3 b6 13. a4 O-O-O 14. a5 Kb7 15. e5 Be7 16. Rd8 Bd8 17. Ne4 Kc6 18. axb6 cxb6 19. Nbc5 Bc8 20. Na6 fxe5 21. Nb4


In this famous game, White did attack on the queenside.  The reason for the exception is the placement of the King.  Opposite side castling throws most chess rules out the window and the only thing that matters is an attack.