After hours trading became available to the retail investor in the late 1990's thanks to computers and the internet going mainstream. After hours trading is basically an electronic market with no regulation, that happens outside of regular market hours (9:30Am - 4:00PM EST). With no regulation means there is no entitlement. This means if a quote is displayed and you had your order in, you may or may not get filled. This is unlike regular market hours where you are guaranteed a fill. After hours is also very illiquid and the spreads are very large. You can only place limit orders during after hours.