Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. d3/4...fxe4/5. dxe4

Ruy Lopez, Schliemann Defence

	a	b	c	d	e	f	g	h
8									8
7									7
6									6
5									5
4									4
3									3
2									2
1									1
	a	b	c	d	e	f	g	h

Common moves:
5...Nf6 - only move

5...Nf6 is the only move. As well as attacking the e4-pawn, it's a case of making the easy decisions first: the g8-knight is the only Black piece that has an obvious square to go to.

Wikipedia has related information at Ruy Lopez.