After-Hours Trading refers to stock trading outside of traditional trading hours of the major exchanges including the New York Stock Exchange and the Nasdaq market. The traditional hours of trade are from 9:30 a.m. to 4 p.m. Eastern Time.
With the rise of Electronic Communications Networks, or ECNs, everyday individual investors can gain access to the after-hours markets. Warning of additional risk  involved in the after-hours period of trade, the Securities and Exchange Commission notes on a page on its website dedicated to the issue of After-Hours Tradingthat while after-hours trading promises greater opportunities and convenience for individual investors, the after-hours market can be much more volatile and far less liquid.
- Trading Stocks After Hours is Risky. About.com.