Learning how to move is one of the first important things that you master during your early developmental years. Whether crawling, walking, running or riding a bike, you develop a "movement philosophy" regarding the best way to move your body around efficiently. This makes the techniques involved in running and biking second nature. As an adult, you don't need to spend a lot of time thinking about why you're able to run or bike faster, you just move your legs faster.
When it comes to swimming, people often assume that they can apply this same movement philosophy to their stroke, only to find that merely moving faster doesn't have the same effect in the water as it does on land. This is because swimming requires a slightly different philosophy than running or biking, and also explains why out of shape swimmers tend to swim as fast, if not faster than, athletes from other backgrounds and higher fitness levels.
The major difference between running, biking and swimming is in the application of leverage. In running and biking you're constantly in contact with the ground, so you can leverage your body forward against that position. This means that you can keep the turnover of your legs fairly high and still be constantly pushing forward.
In swimming, you don't have contact with the ground so you have no leverage other than what you can create with your stroke. If you only focus on moving your arms as quickly as possible, you'll likely veer towards using a "windmill" type stroke.
A windmill stroking movement looks just like it sounds—the swimmers arms move around in a circular pattern at a high rate of speed. This movement tends to make it difficult for swimmers to articulate their arms into positions that best create leverage against the water.
This stroke style also throws swimmers' hips off balance, requires more energy than other stroking styles, and can lead to repetitive motion injuries in the shoulders and elbows.
An easy way to tell if you're using a windmill stroke is to swim a fast-paced 50, paying attention to how you coordinate your arms to one another. If you find that your arms don't move in a coordinated way, you're probably using a windmill stroke.