A service provider is a company that provides you with communications services. Quite commonly, this refers to internet access. For this reason, they’re also called Internet Service Providers, or ISPs.
They do offer other services too, such as short or long distance network connections between buildings, voice and cell phone services, web and mail servers, and more.
Some service providers focus on residential customers, while others focus on business and enterprise customers. Some do both, or have subsidiary companies to specialise in these areas.
Service providers usually put a large investment into infrastructure. This includes running fibre optic cables in the ground, or sometimes through the sea.
A key characteristic of the Service Provider network is providing specialised technology, like MPLS, so they have many customers sharing their physical infrastructure.