Circling is the act of moving around your opponent to reduce damage taken.
Original Author: Pimgd, Original idea: Gunny
When: When engaging in close range combat
What: Moving (partial) circles around an opponent
Why: Close range combat is nasty, and tends to hurt a lot. By moving circles around your opponent, you can throw off his aim, reducing the amount of damage you take.
How: Whilst spraying bullets at your opponent, start strafing until you are very close, then walk (whilst shooting, although you could sprint during reloading, (see Sprint Reloading) circles around him. It won't be possible to dodge all the bullets, but you can most certainly dodge some!
Bonus: You can also try jumping. Be aware that this will also reduce your accuracy, so it's kind of a tradeoff. Still, switching between jumping and crouching is going to take someone's aim off your head.
How to practice: Get a friend to try and kill you with 1 magazine. He'll stand stationary at first, and you can move around him whilst he shoots you. He will used aimed mode at first (hard to aim at you), and later he'll switch to non-aimed mode. For some expert level practice, try a shotgun.