6. Be3, the "English Attack" (for the English GMs such as Short and Nunn who forged into a potent attacking weapon in the late 1980s) is probably considered to be the sharpest line against the Najdorf. Certainly there are people who stick with the old 6. Bg5 or with 6. f4 or 6. Be2, but that is more a matter of personal preference as 6. Be3 continues to be the choice of the top GMs and amateurs alike.

White's plan is similar both in setup and in nature to the Yugoslav Attack in that is both very crude but very dangerous. White usually continues with f3, Qd2, O-O-O g4 in some order, though he must be careful not to allow black to play ...d5 too early.

Black has 3 main replies to the English Attack: 6...e6, e5, or Ng4. 6...e6 was long considered standard but more recently the other two have risen to prominence. \

In the 6...e6 lines, black usually doesn't castle and will usually play Rc8, b5, Nb8-d7-b6 (or c5) and then Nf6-d7 and redeploy the knight at a more active spot. When black doesn't castle, white's plan can often revolve around opening up the center. White can develop Bd3 and Rhe1 when possibilities of a Nc3-d5 sacrifice come into play.

With 6...e5, Black aims for a quick ...d5 with 7. Nb3 (the only try for an advantage) Be6 8. f3 Be7 9. Qd2 b5 10. O-O-O and black will sometimes even play Qc7 and Rd8.

6...Ng4 has frequently been Kasparov's choice with a sample line being 7. Bg5 h6 8. Bh4 g5 9. Bg3 Bg7 10. Qd2 (10. Be2 is also playable, but certainly not 10. f3?? when black has the sneaky 10...Ne3! 11. Qd2 Bxd4 snaring a piece thanks to the fork on c2.) 10...Nc6 11. Nb3 Be6.

Hope that gives you at least a cursory outline, there are plenty of posts on this forum regarding the English Attack and if you subscribe to ChessPublishing.com, GM John Federowicz frequently deals with this popular line in his monthly Open Sicilians section.