Bracing is one of the ways we prevent ourselves from tipping over in a kayak.  There are two basic types of braces, the high brace and the low brace.  Where possible, try to use the low brace as it is much safer.  Several paddle strokes start with the low brace, most notably the back stroke and the reverse sweep.  Paddlers that properly surf holes heavily use the low brace to establish their balance.  The low brace uses the back side of the blade, knuckles down.  The low brace quickly taps the surface of the water with the back side of the paddle blade.  As with all braces, return your head to the upright position last.  Use your hips to rotate the boat upright.  Low braces work best on the front half of the boat.

The key safety item to watch out for is stay inside the paddlers box - don't let the rear blade get out of sight.

Here is a great article on shoulder safety: Shoulder Safety.

Here is a short video on the brace strokes from Mike Aronoff of CKAPCO: Bracing Strokes.

Another great video from Janey Cowie of Zoar Outdoors: High & Low Bracing.