What is the difference between the different kinds of blocks?

Solution 1:

There are some specifics when it comes to blocking.
You have your usual standing block (just holding back), it will block mid attacks and overhead attacks, but won't block low attacks.
And you have the crouch block (hold back and down), this will block mid attacks and low attacks, but you'll be wide open for overhead attacks.
You should be able to tell which "height" property an attack has just by looking at the character animations:

• Mids come out when the character's in a standing position

• Lows usually come out when they're crouching, although there are some exceptions

• Overheads may have some sort of wind-up before they strike, aerial attacks will register as overheads as well.

Blocking will not save you from being thrown
The game also has a tutorial mode where it teaches you these and other basic mechanics of the game.