A period of trading activity that occurs between 4pm(Closing Bell)-8pm. However, your brokerage may only offer after hours trading until 5pm. Institutional Investors have full access until 8pm.
"After-hours trading volume in specific stocks often surges upon the occurrence of market-moving events, such as earnings reports, pre-earnings announcements or M&A activity. Lower liquidity and wider bid-ask spreads are a common feature of after-hours trading. However, investors may consider this a small price to pay for the privilege of exiting a losing position before regular trading commences, or initiating a new position ahead of the crowd. After-hours trading is heaviest in the first hour or two after markets close, before tapering off sharply. As financial markets become increasingly integrated with the advent of globalization, after-hours trading is likely to expand going forward."
( http://www.investopedia.com/terms/a/afterhourstrading.asp#ixzz3fsdxgiss )
