The Ruy Lopez opening is characterized by the moves:
[fen ""] 1. e4 e5 2. Nf3 Nc6 3. Bb5
Why is this opening so popular? It does not appear to be particularly special/unique for white or black, so why is it a popular opening?
The Ruy Lopez is pretty much the epitome of opening principles. White opens with the king pawn to claim the center, and black counters in the same way. Then white develops a knight (knights before bishops!) to attack black's pawn. Black defends the pawn with a knight. Finally, white develops a bishop to b5 where it works in harmony with the knight to attack e5. White prepares to castle at the same time.
So looking at the Ruy Lopez from that perspective, any other moves would violate some opening principle. Obviously there are plenty of openings that are played, but the Spanish Game is one of the oldest and one of the best.
There is a more subtle issue here. Why is
3.Bb5 more popular / better / more interesting than the Italian game
In the Ruy Lopez, white isn't really going to win the
e5 pawn, and the bishop is anyway going to be kicked back to the
b3-f7 diagonal giving black the free moves
b5, so why not play
Bc4 in the first place?
The answer is that the bishop is tactically more vulnerable on
c4, often making an equalizing move sequence
Nxe4 followed by
d5 feasable for black. In fact, due to this important difference, in the Ruy Lopez white always has time to build an ideal pawn centre with
d4, while in the Italian game this is more likely to lead to equality. So in the Italian game, white often has to be content with placing the d-pawn on
More mysteriously, Carlsen has recently started to play the move
d3 in standard Ruy Lopez lines. You have to ask a 2700+ player to explain that one :)
I use the Spanish system on a daily basis. During my early years, I have been told that the Spanish system is one of the most complicated systems and should be learned at a later stage of the chess development ladder. Yes, technically, the white light squared bishop has four squares to choose from after 1.e2-e4, namely e2, d3, c4 and b5. Bf1-e2 is not so popular because it is passive. Bf1-d3 hinders the d-pawn to advance. Bf1-c4 is popular but you have to be an expert to avoid certain variations that give black comfortable play. Hence, we arrive at Bf1-b5.
[FEN ""] 1.e4 e5 2.Nf3 Nc6 3.Bb5
So, what is the major difference between Bf1-c4 and Bf1-b5? That is the real question. First off, black cannot challenge early on using d7-d5, since the bishop is not on c4. Second of all, the bishop on b5 will pin the Nc6 to black's king as soon as black's d-pawn steps forward. Next, black can chase away the bishop using a7-a6 and b7-b5. Yet, this will place the black b-pawn on b5 and allow white the resource a2-a4 and axb5 to open up the a-file and create a target on b5.
The general plan for white in the Spanish system is to keep all the pieces on the board and slowly build up for an attack on the kingside or queenside or both. A typical idea is Nb1-d2-f1-g3 or -e3 to get the knight ready for attack. Another typical idea is c2-c3 to park the bishop on c2 with Bb5-a4-c2, which allows white to keep the light squared bishop from being exchanged off. In summary, the Spanish system is popular because it allows white to have a prolonged and complex battle for the initiative.
The Ruy Lopez is a "basic" opening. It was among the first to be invented and "standardized," first by Sr. Lopez himself, then by Morphy, Steinetz, Alekhine, and others, who discovered variations.
It involves both sides moving out their e pawns, then knights, in accordance with opening principles. Then white attacks Black's c knight with his bishop, Black parries with
...a6, etc., and the battle is on in a natural flow.
Of course, you can use other openings involving other center pawns (e.g. the d pawns), other knights (White plays
2. Nc3, or Black plays
2... Nf6), but those are actually more complicated than the Ruy.