Bb5 in opening charge...
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Study Morphy's Defense, move 3. I believe that that is the predominantly recognized counter to the 3.Bb5 move, which attacks the knight. This is also my favorite counter to the standard Spanish (Ruy Lopez) opening.
In Ruy Lopez, after a6, I capture the knight (Exchange variation of Morphy defence). For me it depends on my mood.
If you look at a knight on c3, c6, f3, f6, you will see that they attack the center and do quite a good job at it. They defend your own pawns or attack enemy pawns. Controlling the center is an important aspect of the opening and a move like Bb5 pinning the knight is strong because it limits the knight's influence on the center. After a move such as a6, kicking the bishop away, the bishop will go to a4, if kicked away again by b5, it will take it's place at b3 and have a strong diagonal aiming and the castled king (as he is usually there).
The case of the pin on f6 or f3 knight is a little different, the pin is also very powerful because it also limits the knight's influence on the center, however you have to watch out because the pin is not full-proof, a knight move can be made, sacrificing the queen for an attack on the king (although rare). Since this time the bishop is on the king side the move h6 or h3 can be played, the bishop will this time retreat to h4, if the move g5 is played a big weakness is created in the castle and white can sacrifice the bishop for the weakness or move it away if possible. The pin is powerful because it is hard to get rid off, getting rid of the pin with pawn moves involves a weaker castle and getting rid of the pin by moving the queen is not available soon because a rule of thumb is to activate the queen after the minor pieces.
You can also defend against both pins by simply exchanging the knight with the bishop.
Also watch out for traps, as with any openings.
Bb5 is good in the ruy lopez because it can become difficult for black to meet and black can find himself restricted. He cant rely on the knight to defend something because white can take it whenever he likes. This is why a6 is often played, kicking the bishop away so that later b5 can be played to protect the e5 pawn. after a6 in the ruy, Ba4 is best. Why exchange your best piece for black's awkward knight, and remove all the tension in the position. This is rather crudely explained though. A move could do all of these things and still be a bad move if for some reason it simply didnt work. Chess is more about the bigger picture, and Bb5 is good here because it gives white a very good game. Some people go to extraordinary lengths to avoid having to play against the ruy lopez, including the petroff, or even playing d4 on the first move 
I believe somebody posted on a subject earlier, where said person refuted the Bb5 pin on the Knight with the Legal's Trap. Examine that too. I believe it is what Manipulated is referring to.
you dont necessarily need a checkmate to unpin your knight. Sometimes you can pinch a pawn with your knight while attacking the opposing queen at the same time, and if the opponenet plays pawn takes knight, you play queen takes bishop.
Unbeliever wrote: I believe somebody posted on a subject earlier, where said person refuted the Bb5 pin on the Knight with the Legal's Trap. Examine that too. I believe it is what Manipulated is referring to.
Légal's trap is on Bg4. It involves a queen sacrifice leading to a fast mate with minor pieces, but it relies on a blunder from black. It will only be a mate if the queen sacrifice is accepted else it involves the loss of only a pawn or sometimes a minor piece.
The full Ruy Lopez against the Standard Morphy's Defense.
1. This is a good question. You do not "lose a tempo" when your opponent plays h6 and you play Bh4, or when your opponent plays g5. Both cases are tempo-neutral - both sides have moved once. A bishop on g3 can be powerful.
2. h6 is a small weakness, and g5 can be a larger weakness. Points to consider: where are both sides planning to castle? can h4/a4 be played to attack the weakness?
3. Sometimes you have to consider which diagonal the bishop is on. For example, Black likes to play h6 in the Leningrad variation of the Nimzo-Indian so that the White bishop (which is now on a different diagonal, h4-d8) will never be able to return via the c1-h6 diagonal to protect the White queenside.
4. Keep an eye out for weak squares, weak color complexes, and things to attack after h6/g5 or a6/b5. But don't rush to do it; more than likely its a permanent weakness that isn't going anywhere. Feel free to strike back in the center first.
I am relatively new to the game and this seems to be a common move in the opening (especially Ruy lopez,Bb5, but others also). How do you defend against the ensuing end pawn (a/h6) defense? Do you retreat? I have always felt if I retreat along the path I cam I lose tempo onto a position that is not as good, so why make the original move at all? and if I move to a/h4 inevitably you have to deal with b/g5 forcing full retreat and losing tempo, I think. Why is this move so common? Is black weakened later in the game by this pawn move which I have not exploited yet?
I have not been able to find a great answer in books (granted I am not always sure where to look) as many openings seem to end with this move! How frustrating! lol.
Thanks in advance for the help,
Joe