Sleet occurs when snow falls from a cloud and melts on the way down. Also when rain falls from a cloud towards a ground or lower atmosphere cold enough to freeze it. A front is a boundary between cold and warm air masses. A cold front means that a cold air mass is advancing towards a warm air mass. The cold front lifts the warm air up, which cools the air, and depending on the relative humidity, produces clouds and eventually rain.
Most precipitation is found around the cold front.
The cold front is depicted on a map by a line with blue triangles pointing in the direction of which the front is advancing. A warm front is the opposite of a cold front. A warm air mass is advancing on a cold mass. Warm Fronts are associated with clear skies and warm temperatures. Warm fronts are shown on a map by a line with red semi-circles pointing in the direction the front is moving.