Walmart (historically Wal-mart) is a discount American department store. They are best known for their exceedingly low prices and dubious business practices.
Founded in 1962 in Arkansas, Walmart is now an enormous multinational corporation, with over 8,500 stores in 15 countries. By revenue it is the world's largest corporation, and the largest private employer with over 1.5 million employees.
Walmart's low prices and wide selection often cause smaller, local retailers to go out of business when a Walmart store is opened.
Similar retailers include Target, Sam's Club (owned by Walmart), Costco, and K-mart.
For more on the history of Walmart, check out the Wikipedia page.