This specific slogan was first launched in 1993 to encourage consumers to drink more milk. Many of the ads that have used this slogan featured celebrities with milk mustaches, or individuals trying to swallow sticky food items that were in need of a refreshing drink to wash it down.

This ad has been hugely successful. According to Gotmilk.com, "Got Milk?" is recognized by 90% of Americans, and continues to be used by the National Dairy Promotion and Research Board (NDB), formed in May 1984 under the authority of the U.S. Department of Agriculture to promote using milk all throughout America.

The phrase itself was created by the U.S. advertising agency Goodby, Silverstein & Partners.

More Info: en.wikipedia.org