The first swimming stroke most of us learn is freestyle, aka the crawl. Most of us still "crawl" along as we swim our laps, but learning a little technique goes a long way - literally.
When swimming freestyle your body should roll and rotate along the axis of your spine. Try this image on for size: you are a screw drilling through the water, not a sled being pulled by just your arms.
When your right arm is in front of your body, about to pull back or stroke, your torso should be rotated to your right side and oriented toward the bottom of the pool (or lake if you swim in nature - lucky you). So the right "edge" of your body, from shoulder to hip is pointed downward and the left side is up. Rotating like this will help make your strokes more efficient and your swimming faster!
Fit's Tip: Alternating your breathing side will greatly facilitate this rolling action. For tips on that, check out: Learn to Love: Bilateral Breathing.