The Pirc Defense starts 1.e4 d6 2.d4 Nf6 3.Nc3 g6. Black fianchettoes the bishop to g7, castles and only then challenges White's centre with ...e5 or ...c5. It is a hypermodern defence: Black controls the centre with pieces first and pawns later.
White has a wide choice of setups, from the quiet Classical (Nf3 and Be2) to the aggressive Austrian Attack (f4) and the 150 Attack (Be3, Qd2 and Bh6). Against all of them Black's task is the same: strike at the centre before White's space advantage turns into an attack.
The Pirc is closely related to the Modern Defense (1...g6), which delays ...Nf6, and to the King's Indian, which arises after 1.d4 Nf6 2.c4 g6.