A warm front is defined as the leading-edge of a warm air mass, which moves forward as a "front", slides on a colder air mass and replaces it by driving out as it passes.
Contrary to the cold front, the border between the two air masses is less visible as it occurs on a larger distance.
Origin of a warm front
The origin of a warm front is due to the tropical air meeting the colder polar air. With the warm front coming, the atmospheric pressure decreases while the temperature increases, because of the warm air elevation. The warm front is more sloping than the cold front and thus leads to different types of clouds and amount of precipitation. Layer clouds, like cirrus or stratus, usually appear above the warm front.
The warm front moves slower than the cold front; that is why we sometimes have an occlusion, when a cold front catch it up. The warm air is then pushed up and a strong depression (low pressure) forms just before.