Both have their place, but dynamic stretches are best.

Static stretches require you to hold a certain position, usually for about 15-30 seconds. Static stretches are best if you want to improve your flexibility.

Dynamic stretches require you to do a continuous movement, and can help to warm-up your muscles. These should be done for 15-30 seconds too.

Dynamic stretches are best BEFORE a workout, because they can relieve tension, prepare the muscle for an exercise, and help you lift the weight easier and avoid INJURY.

Related Questions:

1. Will stretching stop me from getting injured?
2. Should I stretch before my workout, after my workout or both?