A front is a boundary between air masses that leads to a low pressure system as the atmosphere attempts to even out the temperature contrast. If there is enough moisture in the air, the uplift as these two air masses collide causes rain. If the temperature differential is large enough, the collision leads to thunderstorms, according to NOAA.
As a warm air mass collides with a cold air mass, the warmer air is less dense and rises to a higher altitude. As it reaches the higher level, it cools, resulting in a lower dew point. The cooling causes the formation of precipitation, most of the time in the form of rain. However, if the temperatures are cold enough, the moisture crystallizes to form snow. If the advancing cold front is above the ground, precipitation forms before the temperatures change at ground level.
During certain times of the year in the United States warm fronts overtake cold fronts. In these cases, the warm air still rises over the cold, bringing precipitation, but as the cold front retreats, the warm air drops to increase temperatures at the surface of the Earth.
Some warm fronts stall and become stationary fronts. These slow-moving weather patterns dominate what happens on the ground for several days. The weather pattern remains the same until a stronger front overpowers the stationary front to bring change.