Trap: 4...Nxe4?? 5.Qa4+

If Black takes on e4 with the knight, the check Qa4+ attacks the king and the e4-knight at the same time.

1.e4 c5 2.c3 d6 3.d4 Nf6 4.dxc5 Nxe4 5.Qa4+ Bd7 6.Qxe4

Move by move

1.e4 – White occupies the centre and opens diagonals for the queen and the f1-bishop.
1...c5 – The Sicilian Defence: Black controls d4 with a flank pawn and creates an imbalance.
2.c3 – The Alapin: White prepares d4 so that he can recapture with the c3-pawn and keep two pawns in the centre.
2...d6 – Black prepares ...Nf6 without allowing e5 with tempo, but weakens the a4-e8 diagonal.
3.d4 – White occupies the centre with two pawns, as intended by 2.c3.
3...Nf6 – Black attacks e4, which now cannot be defended by Nc3.
4.dxc5 – White takes on c5 and opens the d1-a4 diagonal for the queen: now taking on e4 is dangerous.
4...Nxe4 – A mistake: the knight grabs a pawn but on e4 it is undefended, on the same rank the queen can reach from a4.
5.Qa4+ – A double attack: the queen checks on the a4-e8 diagonal and hits the e4-knight along the fourth rank.
5...Bd7 – Black must parry the check: any interposition leaves the e4-knight hanging.
6.Qxe4 – The queen captures the knight: White is a piece up for a pawn.

White has won a piece for a pawn: the trap works because after ...d6 and dxc5 the a4-e8 diagonal is open and the e4-knight is undefended.

With the extra piece, develop quickly (Nf3, Be2, O-O) and trade pieces: every exchange brings a won ending closer.
Black will try ...dxc5 and ...Nc6 to get active: you need not hold the c5-pawn, the piece is what counts.
If you play Black, remember: after 4.dxc5 recapture 4...dxc5 (after 5.Qxd8+ Kxd8 the ending is playable), not 4...Nxe4.

Qa4+ checks along the diagonal and attacks e4 along the rank: after the interposition the queen takes the knight.