Blocking is a mechanic that allows players to mitigate or outright nullify the damage (or other negative effects) that is incurred whenever an enemy's attack successfully strikes the player. Blocking is featured in games of nearly every genre, but is arguably most prominent in fighting games.
2D Fighting Games
In both 2D and 3D fighting games, there are at least two basic types of blocks: standing block and crouching block. The primary difference between the two sub-genres are the attack "heights" and how each block type interacts with them.
In the vast majority of 2D fighting games, the player blocks by holding the direction (left/right) opposite of their opponent. There is one notable exception that instead uses a dedicated Block button: Mortal Kombat.
High, Low, and Overhead Attacks
2D fighting games have three attack heights: High attacks, low attacks, and "overheads." High attacks represent the vast majority of a character's moveset, while Low attacks are attacks that target the opponent's feet. Overheads consist of a character's aerial attacks and any "command normals" or special moves that specifically target a crouching opponent's head.
High attacks can be defended with either a standing block or crouching block, but the latter two attack heights require a specific block. Low attacks must be blocked with a crouching block, while Overheads must be blocked with a standing block.
In order to punish the excessive usage of blocking, several 2D fighters include mechanics that punish the player for each block. The most prominent method of breaking an opponent's guard is Throwing; both normal throws and Command Grabs are completely unblockable. Chip Damage is another near-universal anti-blocking mechanic that causes the blocking player to incur a small amount of damage whenever they defend against an attack. Essentially every special move in 2D fighting games deal chip damage, but some fighters also allow normal attacks to deal chip.
Another commonly-used mechanic is the Guard Meter. Each blocked attack lowers the Guard Gauge, and the player enters a "Guard Crush" state when the gauge is emptied. Guard Crushed opponents are often left vulnerable for a short while.
Cross-ups are attacks that must be blocked in reverse. These most commonly take the form of aerial attacks whose "hitboxes" are abnormally wide, allowing them to hit the back of an opponent's head. These attacks likely began as an exploitation of hitboxes' imprecision, but have since been taken into consideration by fighting game developers. Mortal Kombat has no cross-ups, as the player is not required to use a directional input to block.
Some games, most notably games with Assists, feature High-Low Unblockables. High-Low Unblockables consist of a simultaneous overhead and low attack that can theoretically hit the opponent on the exact same frame. The player cannot block low because of the overhead, nor can he or she block high due to the low attack; thus, the assault is unblockable. Though true High-Low Unblockables are virtually impossible to create in a non-controlled environment (one frame is equivalent to 1/60 of a second), attacks that hit within two frames (1/30sec) or three frames (1/20 sec) create an assault that is near impossible for a human to correctly block. A few games, such as Street Fighter x Tekken and Skullgirls, prevent this; upon correctly blocking one type of attack, the player's guard will very temporarily block attacks of all heights.
Some fighting games, including the Street Fighter Alpha series and games with aerial-focused gameplay, allow players to block in midair.
3D Fighting Games
Like 2D fighers, 3D fighters feature high and low blocking. However, since 3D fighters have a comparatively minimal aerial game, the three attack heights take on a slightly different form: High, Mid, and Low. High attacks are attacks that target the opponent's head but will completely miss a crouching opponent, mid attacks are attacks that target the opponent's body and cannot be blocked while crouching, and low attacks target the opponent's legs and cannot be blocked while standing. Most movesets in 3D fighting games consist of an even distribution of high and mid attacks, with comparatively few lows.