Cargo can be transported by passenger, cargo or combi aircraft:
Passenger aircraft use the spare volume in the airplane's baggage hold (the "belly") that is not being used for passenger luggage - a common practice used by passenger airlines, who additionally transport cargo on scheduled passenger flights. - this practice is known as Belly Cargo. Cargo can also be transported in the passenger cabin as hand-carry by an “on-board courier”.
Cargo aircraft are dedicated for the job - they carry freight on the main deck and in the belly by means of nose-loading or side loading.
Combi aircraft carries cargo on the main deck behind the passengers’ area with side loading and in the belly.