The majority of all stock trades made in the US occur between 9:30 am and 4:00 pm Eastern Standard Time. However trades can be made after the exchanges are closed if one can find a market maker willingly to accommodate them.
Historically only high net worth individuals and institutions participated in AHT, however now with invention of Electronic Communications Networks (ECN) retail investors have unprecedented access to AHT.
AHT can be risky, as there is typically greater volatility and significantly less liquidity available due to a dearth of buyers and sellers. Additionally there may be less quotes available to individual traders if their ECN doesn't communicate with other ECNs. Accordingly investors who participate in AHT may wish to consider utilizing limit orders to minimize their risk of getting an unfavorable price due to a wide bid ask spread.