The jet stream is a narrow river of air high in the atmosphere. At times there can be several types of jet streams. The polar jet brings down cold air. The subtropical jet brings warmth and moisture from the Pacific Ocean. A low level jet can occur at night and help fuel thunderstorms.
Upper level air patterns can help determine placement of hot and cold air masses and surface high and low pressure systems.
Ed Buckner explains more about Jet Streams in this edition of Weather 101.