When the ball swings in the air it is, basically, referred to as swing bowling. The theory about swing-bowling is that when the ball, with one of its surface smooth and the other rough, moves through the air, the aerodynamics applied on the ball makes the smooth surface cut faster through the air and hence makes the ball swing either out or in - depending on which side the smooth surface is facing.
Outswinger is a term referred to a delivery that swings away (out) from the batsman (all references with regards to a right-hand batsman).
To be able to bowl an outswinger the following guidelines should be followed. The shine of the ball should be facing the on-side. The seam of the ball should be, ideally, pointing towards first slip. Along with a high arm action, the follow through of the bowling arm should finish off with the shoulder pointing towards slip. Apart from these basic guidelines, a lot of practice is needed, which will enable one to pick up the finer aspects of outswing bowling.
An outswinger is an important weapon in the armoury of a fast bowler to test the batsmen on or outside the off stump.