An important piece of information that not everyone (who are not broadband experts) understands is the difference between an Internet Service Provider and a Network Carrier.
An ISP is an Internet Service Provider who provides access to the internet.
A Network Carrier is responsible for the actual physical infrastructure along which electrical signals are passed - the fibre and copper cables, the exchanges and the routers.
To illustrate this good example is BT Broadband and Openreach.
BT Broadband provides its users with access to internet services such as the World Wide Web (websites such as Google, BBC, etc.), email, mobile apps such as Instagram, etc.
Openreach is the actual network over which the BT Broadband services are delivered (bits & bytes of data being transmitted over the physical infrastructure of copper cables and optical fibres).
Thus BT Broadband is an Internet Service Provider and Openreach is a Network Carrier or Network Infrastructure Provider.
Openreach is actually a subsidiary of the main BT business which can be a little confusing - however, it is important to realise that quite a lot of other Internet Service Providers other than BT use the Openreach infrastructure to deliver its services. ISPs who use the Openreach network include Plusnet, Zen, Cuckoo Broadband and many others. The process whereby Openreach allows other Internet Service Providers (i.e. not BT) to use the Openreach network is called Local Loop Unbundling or LLU.