Souen- That's a good question. It turns out to be easy to give an answer to someone who's studied a little Statistical Mechanics, but I'll try to give an answer that doesn't assume that sort of background.
Salt (or other solutes, like sugar) can easily dissolve in liquid water. However, taking the solute out of the water and putting it in the gas phase (air) requires a lot of energy. At temperatures around the water boiling point, these solutes stay in the liquid.
Now the total pressure in the liquid and the air at the boundary are the same- otherwise one would push the other into a smaller space. Part of the pressure in the liquid comes from the solutes, not the water. So the pressure due to the water alone is reduced compared to that of pure water at the same temperature. The vapor pressure, meaning the pressure of water vapor that would stay in equilibrium with the liquid, is reduced by the same amount because of the solutes. (I've simplified and approximated a little here, since the pressure doesn't quite break up into separate parts due to the salt and the water.)
Water boils when the vapor pressure of the water gets to be as big as the pressure of the atmosphere. At that point, vapor bubbles in the water can grow. You have to heat the liquid with solutes up more to get the vapor pressure in it to equal the atmospheric pressure, so it has a higher boiling point.
A very similar argument explains why solutes also lower the freezing point. Since the solutes are almost completely excluded from the solid (like from the gas) they stabilize the liquid. A search of this site will turn up some answers about freezing salt water.
Mike W. (and Tom J.)
(republished on 07/24/06)