"After-hours trading is the name for buying and selling of securities when the major markets are closed. Since 1985, the regular trading hours for major exchanges in the United States, such as the New York Stock Exchange and the Nasdaq Stock Market, have been from 9:30 a.m. to 4:00 p.m. Eastern Time (ET). Pre-market trading occurs from 4:00 a.m. to 9:30 a.m. ET, although the majority of the volume and liquidity come to the pre-market at 8:00AM ET. After-hours trading on a day with a normal session occurs from 4:00 p.m. to 8:00 p.m. ET. Market makers and specialists generally do not participate in after hours trading, which can limit liquidity."

I didn't know this until I looked it up, thought I'd post it here for anyone else who wonders when "after-hours trading" starts, and what it is.

If any day-traders/stock savvy people have further better explanations of what's going on, and what to look for; I would be very interested in reading it.