Excerpts of an article on misc.survivalism titled Can you turn seawater into drinking water?, dated 8 Aug 1996, by John L Freiler (firstname.lastname@example.org).
The term is distilled and yes, it is possible. The simplest systems feature a closed area with sea water in it. The top is made from clear plastic. The sunshines in and causes fresh water to evaporate off the saltwater. Since the air inside the still is now warmer than the surrounding air, the fresh-water vapor will condense on the sides of the apparatus. These parts of the apparatus are designed to let the condensate run down into a separate holding area for collection and consumption.
This sounds complicated, but can be fashioned simply: Suppose you find yourself washed up on an island without a fresh water supply. simply dig a hole in the beach until you reach wet sand (but not actually water, that will cause your hole to collapse). Put a bowl in the center of the pit. Cover with clear plastic (of course, you have to be able to find some...) Seal the edges with more sand and place a small weight in the middle of the plastic right over the bowl. You'll get fresh water as long as there's daylight. Keep the system set up overnight though: It's also a great rain catcher.
You can get much more sophisticated than this; Reverse osmosis systems and active distilling. Both have their advantages and drawbacks; the primary one being cost and maintenance.