The Cable Crossover

Here's a pretty good video demonstration of the cable crossover. This is and isolation movement--it isolates the pectoral muscles.

Here's where the advantage of using cables comes in: you have the same resistance throughout the whole movement--from stretch all the way to contraction. With a dumbbell fly the resistance would decrease as you reach the height of the movement. No such issue with cables.

I would recommend using cable crossovers after you've finished your compound movements (bench press, etc).