The Friday after Thanksgiving in which the American retail industry booms as the Holiday shopping season "officially" starts. It is marked by extended store hours, and consumer competetition in order to take advantage of the numerous discounts and specials.
The name comes from the fact that many businesses make gains for their yearly profits, putting it in the "black" ink as opposed to the red ink, which they often operate on in the months preceding Black Friday.
Another theory or application of the name is that in a sense it is the exact opposite of Black Tuesday, in that the retail economy booms as opposed to collapsing as it did in 1929 on Black Tuesday.
Have you seen the sales at Acme Inc. for Black Friday? We need to go to bed early on Thanksgiving so we can get up in time to beat the crowds there.