In 2009, it was announced that Orange and T-Mobile, part of Deutsche Telekom and France Télécom respectively, were to form a new brand, EE. The joint venture was completed in 2010, and the name Everything Everywhere was announced. The new network allowed customers to use each phone network, as well as access benefits such as Orange Wednesdays. Later that year, several dual brand stores were opened under the EE name.
EE is exclusively a 4G network, and was the first of its kind in the UK. Although the likes of o2 and Vodafone have since launched 4G networks, EE still covers the most areas at the time of writing in 2013.
So, if as a T-Mobile customer you are seeing EE on your screen, it is because they are all part of the same brand.