Found only one good link: http://www.nasdaq.com/quotes/premarket.aspx

Pre-Market'

A period of trading activity that occurs before the regular market session. The pre-market trading session typically occurs between 8:00 - 9:30 A.M. EST each trading day. Many investors and traders watch the pre-market trading activity to judge the strength and direction of the market in anticipation for the regular trading session.

BREAKING DOWN 'Pre-Market'

Pre-market trading activity generally has limited volume and liquidity, and therefore, large bid-ask spreads are common. Many retail brokers offer pre-market trading, but may limit the types of orders that can be used during the pre-market period.