Fronts that bring warm air are referred to as warm fronts. As this warm air approaches, it is lifted upward above the cooler air. As the air in the warm air mass rises it expands, causing it to cool down. As it cools, water vapor can condense, creating precipitation.
This precipitation is generally light and forms gradually. Clouds first form in the sky as the warm air is lifted upward, proceeded by thicker clouds, and ultimately some form of precipitation.