a slip turn is nothing more than using rudder (usually opposite)...
remember the AW "bat-turn"? that would be a prime example of a slip-turn.
below are 2 definitions that basically sum it up, altho' there can be more elaborate types. the one i'd use mostly in AH would be when going vertical, using opposite rudder in conjunction with elevator input to basically rotate around an axis losing speed very rapidly.
The Skid
A Skid (sometimes called a "slip") is a lateral slide to the left or right with very little loss of altitude or change in air speed. It is commonly used to foil the aim of an attacker who is parked on your "six" and getting ready to fire upon you. When performed correctly, your plane will bank slightly and slip laterally out of your attacker's line of fire giving you an opportunity to perform another evasive BFM.
Start the maneuver by dipping one wing and applying opposite rudder. The aircraft will skid in the direction of the dipped wing. The entire move should last only about one or two seconds and should be immediately followed by another move such as a Break Turn or Barrel Roll. Like in the Defensive Spiral, the secret to the Skid is surprise. You want to dip the wing and Skid in the flicker of an eye so the attacker is caught off guard. Take advantage of his surprise and be out of Dodge before he can recover.
Slip-Turn
The slip-turn is a flat turn performed exclusively with the rudder. Unlike a normal banked turn, the slip-turn uses no ailerons.
Most aircraft cannot perform an effective slip-turn. However, the Fokker Triplane did not have a vertical stabilizer, and could yaw very quickly with hard rudder applied. Although the Triplane would slip during the turn, losing a great deal of speed, it could reverse direction in about half the time of other fighters performing a normal turn. To execute an effective slip-turn, don't bank your aircraft's wings.
