A weather front is a boundary where two air masses with different temperatures and densities meet. Fronts are major features on weather maps because rising air near them can produce clouds, precipitation, and changing winds.
A cold front forms when colder air advances and lifts warmer air. A warm front forms when warmer air advances over colder air. An occluded front develops when a cold front catches a warm front, while a stationary front remains relatively fixed because neither air mass displaces the other.
The air masses on either side of a front differ in more than temperature. They may also have different humidity, wind direction, and stability. The exact boundary is often a transition zone rather than a razor-thin line, especially near the ground.
A front is not the same as a jet stream. A jet stream is a narrow band of fast upper-level wind, whereas a front is a boundary between contrasting air masses. Fronts are most familiar in the mid-latitudes, where large temperature contrasts commonly develop.