Backdooring (also BDing or BD) is the commonly used term for a strategy whereupon one or more champions will attack the enemy team's structures such as turrets, inhibitors, or the Nexus without the support of their friendly minion wave. In other words, sneaking into the enemy base without having pushed the lane first.
The benefit of the strategy lies in the element of surprise. Heroes have the opportunity to notice when a lane is being pushed and can move to defend it before the enemy reaches a turret, whereas backdooring can allow the turret to be destroyed before the defending team has a chance to react. It is by common sense for the backdoorer(s) to get to the turret via the jungle, however players can also backdoor down a lane if the minions are right up to the turret. If you are losing you can possibly flash over one of the walls around your base because if you move away from the lane you might get spotted. The main aim of backdooring is to destroy inhibitors but can be used on all buildings as long as the backdoorers have a support or a tank. Alistar is a good pick especially when teamed with a pusher like Master Yi or Nasus.
The term "backdooring" was originally coined by players of DotA.
Backdooring is generally seen as a high-risk high-reward tactic, as in order to attack the tower without friendly creeps nearby a hero must tank the tower's damage her/himself. There is also the danger of being alone deep within the enemy's territory, so the ability to escape in the event of being attacked is reduced. Turrets also take half damage from enemy heroes when there are no enemy creeps nearby, a mechanic specifically designed to make backdooring a turret more difficult (inhibitors and the Nexus do not have this trait).
If a team expects their enemy to attempt a backdoor they will often leave one or more champions to ambush any would-be backdoorers. Doing this without a backdoorer, however, may hinder the team's efficiency in attack.
Abilities with long range teleports such as Twisted Fate's Destiny and Pantheon's Grand Skyfall are particularly suited to backdooring as they can quickly teleport to an enemy tower when they know enemy champions are unable to defend it; their teleports also make them excellent at preventing the enemy from backdooring. Stealth champions such as Twitch and Evelynn are also good backdoorers due to their ability to enter the enemies territory unnoticed, and escape with stealth once the backdoor is complete. In general, a champion that wants to backdoor needs high survivability, since they have to tank a lot of damage, and high DPS, since he has to destroy the turret before the enemies can reach him.
Backdooring is considered unsportsmanlike by many players and is usually frowned upon by casual players, some players actively "ban" the strategy when playing their own custom games. The majority of players, however, accept backdooring as being a legitimate strategy and there are no bans against it in matchmaking or ranked games.