The abbreviation MVNO means Mobile Virtual Network Operator, as opposed to a real Mobile Network Operator (MNO) as e.g. Verizon Wireless, Vodafone, T-Mobile, and many others around the globe. Real MNOs provide part of their network functionality to the MVNO to in turn enable them to provide their virtual network to their own customers, the mobile phone users. The motivation of the MNO to do so is usually to generate additional traffic on their network which is attributed and being paid for by the MVNO. Sometimes MVNO's access to the MNO infrastructure is also determined or even enabled by regulatory rules.
The degree to which an MVNO makes use of its MNO host infrastructure sets the different types of MVNOs that can be found in the marketplace. Originally the V=Virtual in MVNO meant that the MNO provided its radio network only to the MVNO. Characteristic to all MVNOs is that they do not own licensed radio spectrum but take somebody elses.
However, in todays marketplace there are a number of resellers that are also called MVNOs. These marketing-only MVNOs carry their own branding, but they make complete use of their carrier's network infrastructure, both for the wireless and the fixed network part. While the classical service providers use only a co-branding, the marketing MVNO approach makes its customers believe they are using an independent service by creating their own branding. Both however, as many other service providers, are technically completely dependent on their network operators in the area of pricing and billing.
A true MVNO uses the radio network of an existing Mobile Network Operator. Itself, it does not own licensed radio spectrum. Depending on the contractual relationship with its host radio network and the regulatory framework, it operates its own network intelligence, it has its own numbers, and it therefore has some ability to create its own mobile service offerings much more indepently than its marketing-only counterpart. An historic example for a true MVNO was QUAM in Germany, which once had set up itself to start as a true MVNO while planning to build up its own UMTS-based network later. Due to the ruinous UMTS spectrum auction in a number of European countries including Germany, QUAM's operations were terminated later by its shareholders shortly after its start as an MVNO. While true MVNOs are technically able to provide all services allowed by the - standardized - radio host network, the marketing MVNOs are usually limited to "plain old" mobile phone services.
Aside from the mass-market oriented MVNOs, it is possible to build special purpose networks using an MVNO approach. Theses special purpose networks within totally or partially existing infrastructure provide an ideal and very cost-effective communication framework e.g. for public purposes, like police, rescue, or fire fighters communications.