Short question but long answer 
My opinion is there are 2 main lines starting with 5.f4 and 5.Nce2.
There are other moves but the main lines give the main plans and ideas. I had the pleasure to play last week the Haldane Hack (5.Qh5) as Black in a blitz game at Phuket Chess club (www.southernfriedrice.com) vs Mr Smith (who told me he wrote a paper about the line in british chess magazine some years ago). I was crushed in that game but it's blitz and I din't remember the right plan as Black.
Coming back to main lines: 5.f4 c5 (5...a6 will transposes but since c5 is mandatory Black doesn't have to play a6 early) 6.Nf3 Nc6 (6...cxd4!? 7.Nxd4 Nc6 - I don't like Watson's 7...Bb4 with a kind of Winawer without the d4 pawn ; and 7..Qb6?! 8.Be3! Kruppa-Bareev,1988 and a good refutation of the poisoned pawn variation 8..Qxb2? - 8.Be3 transposes) and now White has only two good moves 7.Be3!? (Boleslavsky variation) and  7.Nce2.
For 7.Be3!? you can look at other threads in the forum for information (maybe one game is still running)
For 7.Nce2 (transposition to Classical+Tarrasch) Black has some systems: 7..f6! or 7..Qb6 8.c3 f6 (running into classical positions) ; 7...Be7!?  (Gurevich) ; 7...a6 8.Be3 Qb6 Topalov-Morozevich,Linares-Morellia 2007 subject of last update on chesspublishing
The main line 5.Nce2 is classical system whose exponents are Steinitz,Nimzowitch and nowadays Anand. The Pirc line starts with 5...c5 and now White can delay c3 (to avoid cxd4-cxd4-f6) with 6.f4 (6.Nf3 cxd4 7.Nexd4 Nc6 8.Nxc6 bxc6 9.Bd3 Nc5 is a simple way to equalize as Black) and now Black can use the side-line 6...Qa5!? or moves to classical lines (6...Nc6,6...cxd4,6...Qb6,6..Be7)