Too many times in the gym I see people workout without stretching first. Stretching is a huge part of workout out, getting stronger, losing weight and just being a healthier person.
Now most people think stretching is getting into a certain position and holding it for a few seconds. This is called static stretching and is great, but it doesn't prepare you for a workout. Static stretching is great AFTER your workout.
Prework stretches should be ACTIVE, meaning your muscles are moving, not sitting still. These are called active or dynamic stretches. The reason for this in simple terms is that if you warm up by being still, your muscles will be still and it will be harder to activate them when you lift a weight or start running. When you use active stretches you are actually preparing each muscle to lengthen and then contract which is exactly was is done with resistence training. (Sports Fitness Advisor)
Here is a video of some active/dynamic preworkout stretches. Obviusly you don't need to go as far as the man in the example, but attempting a slower variation is recommended before your workout no matter if you are about to get on a cardio machine or about to do resistence training.