A Block, not to be confused with a Parry, is a way of preventing damage done to the character using it. In order to block, one must move in the opposite direction of the foe; they should block automatically when attacked.
Details
Blocking can be done standing, crouching, and - in the Street Fighter Alpha series and Marvel vs. Capcom series - in midair. Whether or not an attack is blocked depends on what angle the attack is coming from and which direction the player blocked.
- If one is attacked via crouching attack by the foe, then the player must do a crouching block -- the standing block will not prevent the attack.
- If one is attacked while standing, crouching or standing blocks are fine.
- Overhead attacks can bypass crouching blocks, as they strike high, despite appearing to aim low.
- If one is attacked via aerial means, a standing block must be used -- crouching will not do anything to prevent it (although it could evade the attack).
Blocking usually has a visual effect (normally blue) different from when an attack successfully connects.
When blocking a special attack (including more powerful variants such as Super Combos), the blocker will receive Chip Damage. Grab attacks cannot be blocked, but in Street Fighter IV, a "technical" may be performed by attempting to grab a foe who tries to do the same, and cancel out both attempts.
Some games such as Street Fighter X Tekken have attacks that can only be performed while blocking such as the aforementioned game's Cross Cancel mechanic.
Street Fighter Alpha 3
In Street Fighter Alpha 3, all characters have a "Guard Gauge" which limits their ability to continuously block. Certain play modes (V-ism specifically) allow a bigger Guard Gauge. In Classic Mode, however, there is no gauge at all, in addition to the lack of Super Combos.
Guard Bypass
In some games, a property called "guard bypass" (also known as "unblockable" exist that functions very similarly to throws depending on properties. Here Ibuki bypasses Ling Xiaoyu's high (standing) guard with her EX Raida. In this game EX Raida can not be blocked standing.
In some games, such as Street Fighter X Tekken, a property exist called "guard bypass" that can break through the blocking opponent. While rare, some of these attacks can only be blocked high (such as Ibuki's EX Raida) and can be blocked normally crouching. Some characters such as Tekken's Nina has an attack that bypasses high and low guard, effectively acting more like a command grab. Guard Bypass tends to have it's own unique visual effect when it successfully bypasses a guard.