Flanking is a Battle Tactic used for both real-world and video game applications. Its most basic function is to push through a specific direction, in order to achieve domination of a strategic location or in order to exploit a weak point in the structure of an enemy's force. In World of Tanks, flanking is used by faster and lighter tanks to hit a slower and heavier tank without much risk of being hit back.
Flank your enemy's weak sides and rear, especially against slower targets. Tanks have heavy armor up front, lighter armor on the sides and are weakest in the rear. A quick medium tank may be able to outmatch a stronger heavy tank by simply outmaneuvering it and firing at its weakest points while avoiding its slowly rotating turret. This is particularly effective in one-on-one combat on open ground where there's little cover.
Don't allow yourself to be flanked - one-on-one. When engaging a single enemy that's trying to flank you, continue to turn and face him while reversing. This will force him to make a wider turn than you, exposing his weaker side armor while he must penetrate your thick front armor. This is especially important when driving non-turreted tank destroyers, which have thick frontal armor and a fixed, powerful cannon.
As seen above, the heavy tank reverses, keeping the turret and frontal armor facing the more dangerous foe.
Don't allow yourself to be flanked - multiple targets / forward threat (more common). When multiple enemies are attempting to flank you or you're about to be flanked while there's the threat of enemy fire from forward your position, simply reverse, keeping your front armor facing the most serious threat (whether that be one of the flanking tanks or a sniping tank directly in front). This will delay the enemy from flanking your rear, allowing you to get shots off in the mean time. Remember to concentrate your damage on one target until it's gone or you are.
When confronting Tank Destroyers, it's best to flank them, as Tank Destroyers usually don't have any turret, and will have to turn their entire hull in order to catch up, and have no turret to aid. Some American Tank Destroyers have turrets, but the slow turret traverse speed makes them easy to flank and destroy.