It's an unofficial retail "holiday." Basically, the day after thanksgiving marks the beginning of the "Christmas season," when it's okay to start decorating/listening to carols/gift shopping, etc. Thanksgiving is always on the fourth Thursday in November, so people with real (read: non-retail) jobs generally take Friday off as well and have a long weekend. Schools also usually aren't open, either. Because everyone is generally home and the holiday season has unofficially begin, stores and other retail outlets post huge sales on popular or expensive items for that day, and open extra early, usually around 4am., and hordes of people show up even before then and line up outside the door to try and get inside first. (Hot items like TVs and popular toys as well as rare items and things with the biggest discounts run out really fast, so people try to get in first so they can get dibs on what they want.) It's called "Black Friday" because the stores usually make so much money that it makes up for earlier periods when they were losing money ('in the red'), bringing them back 'in the black.'

A lot of people think it's a fun way to start the holidays, fighting the crowds and being out in the excitement, hitting good sales, etc, but since I work retail, I think it's the worst thing ever. It's massively crowded all day and all the popular things run out, the store is a mess and it's impossible to get your work done or keep anything in order. Usually there are fistfights, and every year there are news reports of people being trampled in the mad rush when the stores open. Luckily, neither of those happened in our store, just a lot of employees trying to function on an hour of sleep, being unaccustomed to such ridiculously early shifts. It's pretty much the worst American tradition ever.

You'd probably get a more complimentary description from someone else, lol, but I'm jaded.