A network is the system that controls radio signals relayed to and from your mobile phone. In this country there are 6 operators: T-Mobile, 02, Orange, Fresh, Virgin and Vodafone.
Choosing the right network is the first, and one of the most important decisions you must make when choosing a mobile phone. Each network claims about 99% coverage, but there will always be localised "black spots". Each network offers a large range of services that differ from operator to operator, so it's essential to choose a network that is relevant to your needs.