Just about every passenger flight is carrying some freight along with the passengers and their baggage. The U.S. Postal Service alone leases space on 15,000 of the approximately 25,000 scheduled passenger flights each day. Commercial airlines make about 5 to 10 percent of their revenue from hauling freight.
When a package is shipped on your flight, it is usually consolidated with other packages and freight and packed into special containers that fit in the storage area under the passenger compartment. For instance, a Boeing 747-400 (one of the largest passenger planes) can hold 416 passengers along with 5,330 cubic feet (150 m3) of cargo. That's about as much cargo as can fit in two semi-truck trailers.
Most of the freight is stored in special containers shaped to fit themselves to the inside of the cargo hold. Some of the freight is also put on pallets, and loose items may be placed in any open spaces that remain.
The 747-400 can also be configured in "Combi" mode, in which some of the passenger compartment is used to store freight. In this mode, the plane can carry over 10,000 cubic feet (283 m3) of cargo and 266 passengers.
And it can hold a lot more cargo when configured as a dedicated cargo plane.