MVNO stands for mobile virtual network operator. It's the official term used for a cellphone carrier that operates off another network.
These MVNOs buy wholesale voice, text and data minutes from one of the network operators like AT&T, T-Mobile, Sprint and Verizon.
Because they don't own their own network they usually offer really good value plans and prices - they don't have the overheads that the big network carriers do.
The main benefit to customers is that you are using the exact same service and network as you would if you had a cellphone plan with the big network carriers - but you will usually pay a lot less or get more value.
There are two types of MVNO - those that are linked to the big networks, like Cricket (AT&T), Metro PCS (T-Mobile) or Boost (Sprint) and those that are independent.
The only thing to bear in mind is that you may get slightly lower data speed than you would if you paid for a direct plan.
Here's a list of MVNOs that work off each network: