The networks are the key to your experience as a mobile phone user. They will determine if you get good coverage and how good the customer service is. The networks all offer different features.
O2, Vodafone, Orange, 3 Mobile and T-Mobile are networks – they supply the service, not the phone. There are also ‘virtual networks’ such as Virgin mobile, which uses T-Mobile, OneTel, which uses Vodafone, NTL Mobile, which uses Orange and Tesco, which uses O2.