(all directional elements refer to northern hemisphere; reverse them for southern hemisphere conditions)
In weather, a low pressure area is an area which has relatively low air pressure (as measured by a barometer). Low pressure areas are charactarized by rising air, and often stormy weather conditions. Often, in temperate areas, they are associated with fronts, trailing off in a wedge to their south, the warm front to the east of the cold front. In these areas, the lows generally move from west to east in the jet stream. Lows which become disconnected from the jet stream are called 'cutoff lows' and move erratically. In more tropical lattitudes, lows are not associated with fronts, but may become tropical storms or hurricanes in the summer. Sometimes in the desert, thermal lows will form due to rising of very hot air; these produce no precipitation.
In the northern hemisphere, wind in lows rotates in a counter-clockwise direction at the surface. Often the fronts are associated with moisture feeding up from warmer, more tropical areas, giving the low a 'comma' shape. Lows are depicted as large L's on weather maps.