The queen is the most powerful piece on the chessboard and beginners and amateurs are usually all fired up to use it for aggressive intentions. The queen however can also be used very effectively for defense purpose but it is for the player to judge exactly how to employ her (according to the demand of the position).

In the game which follows, Black embarks on a queen sacrifice which exactly cannot be called spectacular. 
White probably is happy with the proceedings and makes a mistake in immediately trying to use his queen for an attack , placing it on the wrong square. Black capitalizes and uses this opportunity to combine his pieces to eke out a nice victory.

White: Ruslan Kashtanov (2325) – Black: Valerij Popov (2534).

St Petersburg, 2000
Sicilian Defence
1.e4 c5.

The Sicilian Defence which is considered a sharp response to the King pawn Opening.
2.Nf3 d6 3.d4 Nf6 4.Nc3 cxd4 5.Nxd4 a6 6.f3 e5 7.Nb3 Be6 8.Be3 Be7 9.Qd2.
The players are playing in accordance with opening theory.
9. ..h5 .

A bold approach.

10.a4 Nbd7 11.a5 Rc8 12.Be2 Qc7 13.0–0 0–0.
Both have castled on the King-side.

14.Rfd1 Rfd8 15.Qe1
If 15.Nc1 d5 16.Nxd5 Bxd5 17.exd5 Qxc2 18.d6 Bf8 19.Na2 Qxd2 20.Rxd2 Rc6 21.Rad1 Nb8 22.Ba7 Rcxd6 23.Rxd6 Bxd6 24.Nc3 Nc6 0–1  was a game played previously.

15...Qc6 16.Rac1.

If 16.Qd2 Qc7.

16...d5.

This should be a triumph for Black for he has managed to push his backward pawn which would have given him trouble later.

 17.exd5 Nxd5
If 17...Bxd5 18.Nxd5 Nxd5 19.Bd2 Qf6.

18.Nxd5 Bxd5.

Black looks better after these exchanges as his pieces have come to the centre.

19.c4 Be6 20.Qc3 h4.

Advancing into enemy territory.

If 20...Qa4 21.Qc2 leads to an equal game.

21.f4.

If 21.Nd2 h3 22.g3 Nf6 with a slight advantage for Black.

21...h3.

He has gone deep into enemy territory.

22.Bf3.

If 22.Bf1 exf4 23.Bxf4 Nc5 (23...Bf6 24.Nd4) 24.Nd4 Qe4 25.Qe3 Bg4.

22...Qxc4.

Good move leading to complications. Black would have held an edge after   22...Qc7 23.g3 Bd6.
23.Rxd7.

If 23.Qxc4 Rxc4 24.Bxb7 Rb4 25.Bc5 Rxb7 26.Bxe7 Rdb8 would give advantage to Black.
23...Rxd7.

If 23...Qxc3 24.Rxd8+ Bxd8 25.bxc3 exf4 26.Bxf4 Bxb3 27.Bxb7 Rc5 28.Bxa6 Rxa5.
24.Qxe5 Bf6.

Making things interesting with a queen sacrifice. He could have attained equality with 24...Qxb3 25.Rxc8+ Bd8 (25...Bf8 26.Rxf8+ Kxf8 27.Qb8+ Ke7 28.Bc5 with a good game for White) 26.Bd4.
25.Rxc4 Bxc4.

If 25. ..Rxc4  26Qb8+ Rd8  27.Qxb7 works out to White’s advantage.

Diagram 1
26.Qh5.

A mistake. In trying to cash in on his queen, White tries to work out an attack on the King-side. However White could have done better with 26.Qf5 Be6 27.Qe4 Bxb3 28.Bg4 Bc2 (28...Re7 29.Qxe7 Bxe7 30.Bxc8 hxg2 31.Bxb7 Bb4 32.Bb6 Bc4 33.Kxg2) 29.Qb4 (29.Qf3 Bd1 30.Qxh3 Bxg4 31.Qxg4 Rdc7) 29...Be7 30.Bxd7 (30.Qe1 f5 31.Bb6 Rc6) 30...Bxb4 31.Bxc8 hxg2 32.Bxb7 Bxa5].

26...Bxb3 27.Bg4 Rd5 28.Qxh3.

If 28.Qxd5 Bxd5 29.Bxc8 Bxg2 wins for Black.

28...Rc2 29.Qf3.

If 29.Qg3 Rxa5 30.h3 Rxb2 .29...Rxa5 30.h3 Bd5.

The finishing touches! White hardly has any counter play left.

31.Qd1 Rxg2+ 32.Kf1 Rh2 33.Qe1.

If 33.Bf2 Bxb2.

33...Ra2.

Both rooks are now patrolling the 7th rank and White is  lost.

34.Qd1 Bg2+ .

If 34...Bc6 35.Bd4 Bxd4 36.Qxd4 Ra1+ 37.Bd1 Rxd1+ 38.Qxd1 Rh1+ 39.Ke2 Bf3+.
35.Ke2 Be4+ 36.Bf2.

If 36.Kf1 Raxb2.

36...Rxb2+ 37.Ke3 Bc6 38.Be1 Rh1 and White resigned.

Diagram 2
White to play and win
1.Rxh6+ Kxh6 2.Qg5+ Kh7 3.Qh5 check-mate.