Maestro is owned by MasterCard and is their main debit card brand in Europe and some other countries (like India). It is the MasterCard equivalent of Visa Electron.
• Maestro is accepted at 12 million points of sale.
• Visa, as of 2009, is accepted at 30 million merchants in 170 countries.
• MasterCard is accepted at 28 million merchants in 210 countries.
MasterCard has better worldwide acceptance than Visa, but Visa has better acceptance in the USA.
The main reason that Maestro has only 12 million merchants is that support in the USA is weak. There are lots of reasons for this, but it comes down to MasterCard focusing on Europe and International due to differences in the PIN debit networks between the USA and the rest of the world.