In chess, one of the main variations of the Sicilian Defence is the Dragon Variation. 

The Classical Dragon starts with the following moves. 

1.  e4 c5

2.  Nf3 d6

3.  d4 cxd4

4.  Nxd4 Nf6

5.  Nc3 g6

In this variation, Black fianchettoes a Bishop on g7 square.  The name "Dragon" was first coined by Russian Chess Master and amateur astronomer Fedor Dus-Chotimirsky who noted the resemblance of Black's kingside pawn structure to the constellation Draco.  Though I can't see it myself, I do believe that The Dragon Variation is one of the sharpest variations of the Sicilian Defence, making it one of the sharpest of all chess openings. 

Dragon Relatives

If you are new to Dragon theory it is worth noting that there are about 3 other Dragon opening systems and they are the following:

1.  Accelerated Dragon - this opening system features an early ...g6 on the 4th move from Black.  An important difference between this line and the Classical Dragon is that Black avoids playing ...d7-d6, so that he can later play ...d7-d5 in one move if possible.  Black also avoids the Yugoslav attack, but since White has not been forced to play Nc3 yet, 5.c4 (Maróczy Bind) is possible.  Generally, this opening features a more positional style of play, unlike many of the other Sicilian variations.  The opening move order for Accelerated Dragon is as follows:

1.  e4 c5
2.  Nf3 Nc6
3.  d4 cxd4
4.  Nxd4 g6

2.  Hyper Accelerated Dragon - is just a move-order difference of the Accelerated Dragon.  Here ...g6 is done earlier on the 2nd move.  The idea is just to avoid the possible 3.Bb5 Rossolimo Attack by White, a serious sideline that also boast a lot of theory.  As a consequence White at his disposal can steer the game to 1.e4 c5 2.Nf3 g6 3.d4 cxd4 4.Qxd4 , which attempts to disrupt Black's normal development.  The opening move order for Hyper Accelerated Dragon is as follows:

1.  e4 c5

2.  Nf3 g6

3.  Dragondorf - also called the Dragadorf, is a hybrid Dragon with the Sicilian Najdorf defence.  In this system Black plays both ...g6 and ...a6.  Theory in this line is still relatively new thus it is yet to be proven if it can widthstand the test of time.