The King of Fighters XIII/Systems/Defensive Actions: Blocking
In The King of Fighters XIII, blocking works the same way as you would expect in any traditional fighting game. So, holding back while your opponent is attacking will block high and middle attacks, but not low attacks, while holding down-back while your opponent is attack with block low and middle attacks.
If you block attack you will stay in 'blockstun', during which you can do anything except alternate between standing block and crouching block or execute a Guard Cancel Blowback or a Guard Cancel Roll. The length of hitstun depends on the attack, it's short for light attacks and longer for hard attacks and special attacks, and the longest for CD attacks.