A low pressure system has lower pressure at its centre than the areas around it. Winds converge towards the centre of a low pressure area, and are subsequently forced upwards as shown in the image below. As the air rises, the water vapour within it condenses, forming clouds and often precipitation.
A low pressure system results in a region of converging and rising air. In the northern hemisphere, surface winds within a low pressure system rotate in an anticlockwise direction around its centre.
A trough is an area of elongated low pressure indicated by isobars extending outwards from the centre of an area of low pressure. The weather within a trough is similar to that of a low pressure system.