being buoyant allows you to stay on the surface of the water easily, but other than buoyancy, you need to have good technique, a muscular build and a confident mind to be a successful swimmer.

Buoyancy in this case refers to the difference in density of the swimmer vs that of the water. The more buoyant you are, the larger portion of your body will be above water when you attempt to float (with no other motion).

Obviously, the more buoyant you are, the simpler it is to keep your head above water and be able to breathe. That is, a more buoyant person will have to spend less energy to keep themselves from drowning.

Buoyancy can come from several areas:

(1) ironically, fat is much less dense than muscle, so a person with a higher body fat percentage (BMI) is much more buoyant than one with a low BMI. Which means that obese people float much better than fit people. However, as obese people are much less able to keep up a sustained level of exercise required to actively swim, a fit person is a much better swimmer.

(2) flotation aids - such as a life vest, inflatable "floaties" and similar objects, or even clothing properly used in a lifesaving manner (e.g. a pair of pants taken off, reversed with legs tied behind the neck then filled with air) can provide a large extra buoyancy measure, allowing the person to remain afloat with virtually no effort. However, actively swimming while wearing such aids is very difficult.

(3) External objects that the person can grasp. Anything from a piece of wood, to a life preserver can increase the person's buoyancy. Once again, this allows the person to stay afloat (so long as they have the strength to grasp the object), but seriously inhibits swimming.

In general, the more buoyant a person is (whether due to fat or a worn or grasped floatation aid), the harder it is to swim properly. "Good" swimmers are much like sharks - they tend to be naturally non-buoyant, but use very efficient movements to maintain forward motion, and thus, remain able to stay afloat.

A good example of the difference between the two: a typical competitive swimmer (not an olympic-class one, just your average high school swimmer) should have no problem treating water for several hours unaided. A typical obese person will generally become tired and begin to founder after several dozen minutes (no more than a half-hour, at best), even though the obese person must put in much less effort to stay afloat. In fact, most competitive swimmers have negative buoyancy, which means that they will sink (even when holding their breath) if they don't actively treat water, while greatly obese people need to do no more than slowly wave their arms just below the water's surface to keep in an upright position.

Click to enlargeHold your legs together and extend your arms above your head.

Click to enlargeKick your legs up and down once in a whipping motion generating from the hips and bending at the knees, as if you were a dolphin.

Click to enlargePull both of your arms simultaneously and symmetrically through the water beneath your body along with the big kick, helping to propel your body forward and out of the water.

Click to enlargeLift your head up and breathe as you quickly pull both arms out of the water and swing them forward. Head and arms reenter the water together in a diving motion.

Glide momentarily, performing a smaller follow-up kick.

Execute another pull-through motion with your arms, with your legs performing a full kick to propel you up and out again.

• The butterfly kick is actually two kicks. One kick is a big kick, with knees bending and kicking out while the body leaps and dives forward. Then, during the glide, the whole body whips the legs into a second, smaller kick.
• Done properly, the butterfly stroke is a smooth, gliding stroke, but you may find it very difficult and tiring when learning.
• Practice the kick without the stroke to get a feel for using your whole body. Extend your arms and undulate your body through the water.
• Try practicing the kick with fins on to get the feeling of a powerful and fluid kick.

Information received from http://www.ehow.com/how_5301_swim-butterfly-stroke.HTML