A bus is a long vehicle with many seats. An elementary school student might save a seat on the school bus each morning for her best friend.
A city bus carries far more passengers than a private car, which makes it an efficient form of transportation. When bus is used as a verb, it means to transport by bus, sometimes in an effort to desegregate a school district: "The city will bus students from this neighborhood to the other side of town." It also means "to clear a table" in restaurant lingo, which is often done by a "bus boy" or a "busser."