Backdooring (also BDing or BD) is the commonly used term for a strategy whereupon one or more heroes will attack the enemy team's structures such as towers, barracks, or the Artifact without the support of their friendly creep 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. Players have the opportunity to notice when a lane is being pushed and can move to defend it before the enemy reaches a tower, whereas backdooring can allow the tower 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 tower via the jungle, however players can also backdoor down a lane if the creeps are right up to the tower. The main aim of backdooring is to destroy towers but can be used on all buildings as long as the backdoorers have a support or a tank.
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.
If a team expects their enemy to attempt a backdoor they will often leave one or more heroes to ambush any would-be backdoorers. Doing this without a backdoorer, however, may hinder the team's efficiency in attack. Alternatively, Sentry Wards can be placed at possible infiltration points to see heroes attempting to backdoor before they reach their target.
Heroes with the Transport talent are particularly suited to backdooring as they can quickly teleport to an enemy tower when they know enemy heroes are unable to defend it; their teleports also make them excellent at preventing the enemy from backdooring.
Backdooring is considered unsportsmanlike by some players and is especially frowned upon by casual players. Some inhousers ban this strategy for it is considered "BM" in their eyes. The majority of competitive players, however, accept backdooring as being a legitimate strategy and there are no bans against it in most, if not all, tournaments.