The main problem when it comes to understanding the internet is that everything seems to have an abbreviation.
ISP stands for 'internet service provider'. ISPs are the companies who provide your broadband connection, whether it's a mobile or a fixed-line home connection.
ISPs and BT
The majority of the UK's landline infrastructure is owned by BT. If you have phone-line internet (also known as ADSL), the provider you get it from rents BT's lines and telephone exchanges but uses its own internet-providing software to give you your broadband connection. This is why you have to pay line rental to BT as well as service costs to your provider.
The only companies that are independent of this are mobile phone providers (who provide 3G and 4G internet connections wirelessly through mobile networks) and Virgin Media, who have their own cable infrastructure (having bought it from the NTL Telewest group in 2008).
Not sure which ISP is best for you? uSwitch Broadband lists the majority of the best UK internet service providers in the UK for you to compare and contrast, so start your ISP comparison here!