Trading taking place after regular trading hours is known as after-hours trading (AHT). Today, many traders have access to electronic communication networks (ECNs) where they can carry out after-hour trading on the after-hours market.
When market moving information becomes public, we can usually see an increase in after-hours trading in relevant securities, since traders do not want to wait until the regular market opens to act on the new information.
The after-hour market will normally have a much lower liquidity than regular markets and you can expect bid-ask spreads to be wider.
The first two hours after market close is usually when a majority of the after-hour trading is carried out.
In the late 1990s, several major U.S. stock market exchanges introduced extended and after-market trading hours. NYSE and Nasdaq are two examples of exchanges that allow institutions to place order interest and trading activity outside of regular trading hours.