A front is the meeting place of two separate air masses. There are four types- Cold, Warm, Occluded, and Stationary. Fronts are associated with the weather in the middle latitudes.
Cold fronts are created when a cold air mass moves under a warm air mass quickly, and pushes it up. They cause thunderstorms, heavy rain, or snow through cumuliform, and are followed by cooler weather.
Warm fronts are created when a warm air mass moves over a cold air mass slowly, and pushes it under. They cause drizzly rain through stratiform, and are followed by clear and warm weather.
Occluded fronts are created when a cold air mass moves under a warm air mass, and then under a slightly less cold air mass and pushes both up. Occluded fronts cause cool temperatures, and large amounts of rain and snow.
Stationary fronts are created when a cold air mass and a warm air mass meet. If there's no wind to move either of them, they can stay there for days. They can cause many days of cloudy, wet weather.