Both Visa and MasterCard are very similar -- American multinational financial services companies, operating branded credit and debit card networks. Their functionality is basically the same.
There are negligible differences in products, primarily in international debit card offerings, between Visa and MasterCard. Visa has slightly higher market share, especially outside the USA, and has generally been somewhat faster at adopting new technology in trials, but there hasn't been a huge difference in deployed products.
Visa, as of 2012, is accepted at over 45 million merchants in 220 countries. MasterCard is accepted at 28 million merchants in 210 countries. These statistics differ depending on source because both companies don't disclose the actual number but arbitrary figures. MasterCard has better worldwide acceptance than Visa, but Visa has better acceptance in the USA.