A mobile virtual network operator, or MVNO, can be an inexpensive alternative for your mobile service. These companies pay to use the radio spectrum that they use but do not own that spectrum. In the US popular MVNOs include Boost Mobile, Virgin Mobile, Straight Talk and Simple Mobile. However, plenty of companies like this exist.
These companies offer contractless service that often costs much less than comparable service with the company that owns the network. For instance, both Boost Mobile and Virgin Mobile run on Sprint's network. Anywhere Sprint has 3G they have service available. Consumers who live in an area where Sprint has no 4G might prefer the convenience of a monthly cell phone plan that costs less.
There are a few things to keep in mind when moving to an MVNO, however. You may not be able to take your current phone with you and these operators often sell phones at a higher price without the contract to offset the price. The exception is when you own a GSM phone that the carrier unlocks. Then you can simply replace the existing SIM card with one from the MVNO, but CDMA carriers like Sprint and Verizon do not use SIM cards, making this impossible.
Keep in mind that if you're a customer of one of these networks you're lower priority which can lead to dropped calls or text message lag. But many consider the inconveniences minor and well worth the savings.