Carrier - definition
Carriers (sometimes service providers, operators) are the companies that sell the use of a wireless network. Usually they own the network though some (called MVNO) do not.
The network consists of base stations (cell towers) and the infrastructure linking them.
The service allows the user to access the network and they are billed by the minute (for calls) or by kilobytes (for data transfers). Such services are sold as packages known as "calling plans".