On May 5, 1862, the Mexican army defeated the French army at the Battle of Puebla. This single military battle signified defeat of a European colonial power, and a victory for the Mexican people. This single battle was the roots of Cinco de Mayo.
Note: Cinco de Mayo is not the celebration of Mexican Independence which is celebrated on September 16th.
What Cinco de Mayo has come to be, is much more than one battle in the colonial history of Mexico. Rather, it has come to signify Hispanic and Mexican pride and a time to celebrate the rich culture.
Today, this holiday is celebrated by Mexicans, and especially the Hispanic community in the U.S. It is a time of song, dance, partying, and in general a time to be proud to be of Hispanic descent.