T-Mobile is a wireless carrier in the U.S. that is currently the fourth-largest in the country by number of subscribers. Known for its token commercial jingle and magenta color scheme, T-Mobile drives itself on the message of being an "Uncarrier" that does things the other carriers just won't do. It's well known as a value-based option to the other three major carriers, with lower prices for data and fewer restrictions on usage than its competitors. Though it is no longer a wholly-owned subsidiary of Germany's Deutsche Telekom, DT now holds a majority of the now-public T-Mobile U.S. stock.
Following a failed attempt to be acquired by AT&T at the end of 2011, T-Mobile gained a huge sum of money and spectrum holdings as part of the deal to the tune of $4 billion. Using this money and a change in leadership at the carrier, T-Mobile quickly turned itself around to start gaining customers and posting small profits for the first time in years. From 2012 to 2014 it launched and rapidly deployed an LTE network, quickly bringing it up to speed in terms of technology and coverage with the other carriers. Part of that expansion came with the purchase of MetroPCS, a smaller regional carrier, which brought in more customers, new spectrum and subsequently made T-Mobile a publicly-held company through the merger.
In terms of network technology, T-Mobile operates a GSM and LTE network across 700MHz, 1900MHz and the combined 1700/2100MHz (also known as AWS) frequencies. During its vast expansion of LTE throughout 2013, T-Mobile shifted its spectrum holdings in a way that better aligned itself with the network map of AT&T, making it easier for customers of that carrier to bring their phones over to T-Mobile without issue. Though it has made huge strides in terms of coverage in recent years, T-Mobile is often dinged — both by customers and competing carriers — for its lack of extensive rural coverage around the country.