[LandR]

Sorry, not American but this seems an interesting aspect of a sport I know nothing about!

Is this done because the catcher might know the batter is worse at hitting pitch type X, but is better at pitch type Y, so signals to the pitcher to throw an X?

But the pitcher might think he can't throw a good X right now (for whatever reason), and instead will throw a Z ?

And the pitcher and catcher need to somehow secretly agree on what's being thrown?

Does it ever happen that the catcher fails to catch it? It seems he has a pretty small window that he needs to catch it in, but then I guess sometimes these pitches are crazy fast and can't have much time to react.

[KevinEldon]

Yes, the catcher does miss the ball occasionally. It's common for the pitches to be 90 MPH or higher. I've seen them as high as 104 MPH. The ball will also move; it's not always thrown in a straight line. Some times it cuts across the plate, other times it can drop as it crosses the plate. If the catcher knows where to expect the ball they have a higher probability of catching it.