Each team has 3 inhibitors on Summoner's Rift in each of their bases, of which at least one inhibitor must be destroyed to render the Nexus and its defending turrets vulnerable to being attacked (the Nexus will be invulnerable as long as one of its turrets or all 3 inhibitors are standing). Every inhibitor is protected by a turret that makes that inhibitor invulnerable; destroying the turret makes the inhibitor vulnerable.
Once a team's inhibitor is destroyed, the enemy's Nexus will begin spawning a super minion in the waves set in the respective lane, replacing the siege minion in its place if applicable. If all 3 of a team's inhibitors are destroyed, it will instead spawn two super minions in each lane, per wave.
Destroyed inhibitors respawn after 5 minutes, therefore stopping the opposing Nexus from spawning any more super minions.
The damage dealt to an inhibitor with a basic attack is the sum of the base attack damage, plus the higher of bonus attack damage and 60% of ability power. If 60% of a champion's ability power is greater the amount of their bonus attack damage, the attack's damage converts from physical damage to magic damage.
When the announcer notifies that either yours or the enemy's inhibitor will respawn soon, said inhibitor will respawn in 15 seconds.
In Summoner's Rift, destroying an Inhibitor will spawn a Super Minion in each wave for the next 8 minion waves. Usually two minion waves before the Inhibitor completely recovers will not have any Super Minions.
In Howling Abyss, destroying the Inhibitor will spawn a Super Minion in each wave for the next 10 minion waves.
Once its protecting turret is destroyed the inhibitor is extremely susceptible to backdooring. An enemy marksman will be able to take down an unprotected inhibitor before a defending champion can Recall and walk to defend it. Additionally, one can't directly Teleport to it. Destroying an inhibitor can often be worth more than a few champion kills because of the immense lane pressure that Super Minions will provide.
League of Legends have had numerous different inhibitors during its history. Most were primarily used for Summoner's Rift while the remaining ones for ARAM, Twisted Treeline, and other game modes.
Context: At the beginning of the 2014 season we shortened the respawn time of inhibitors. Now, with our LASER-BUFFED BASE TURRETS (shouting is mandatory here), we can put inhibitor respawns back to their original timing so that attacking teams have a better window to capitalize on their achievements.
Inhibitor death timers increased to 5 minutes (from 4 minutes).
Context: Losing an inhibitor often puts a team behind for a long period of time with few opportunities for retaliation. This was because a downed inhibitor also granted a global buff to minions in all lanes. We want more comeback opportunities for teams who can capitalize on winning teamfights, even with a downed inhibitor.
Inhibitor death timers reduced to 4 minutes (from 5 minutes).
Inhibitors no longer grant a global buff to all minions when destroyed.
The bonus amount ranged and melee minions receive was previously based on the number of enemy inhibitors destroyed as their wave spawned.
Inhibitors now grant a stronger damage and durability buff to minions in their specific lane.