I guess it depends on what you are trading. For stocks, the exchanges close around 4 pm so any stocks you trade after that are done during aftermarket hours. Not all stockbrokers may support aftermarket hours trading & the ones who do may charge higher fees.
Not every order may be filled during aftermarket hours as some brokers only permit pending orders so that if they can fill it they do and if they cannot fill it, they leave it unfilled.
For forex, aftermarket trading is when you trade outside the 4 major sessions (Sydney, Tokyo, London, New York).
For example, in-between the Asian and the London session, there is a 2-hour gap which some call the Frankfurt session or the Pre-London session. If you trade during this time, it could be considered aftermarket hours trading.
People who participate in aftermarket hours trading are the ones who are busy during the market hours, perhaps they have day jobs that don't allow them to trade when others are trading.
