Liqueurs are a family of alcoholic drink which contain distilled spirits (not just gin!) and additional flavourings; sugar, fruit, herbs and spices are all common.
Generally liqueurs are quite sweet, and served with desserts. This is not to say they can't, or shouldn't be drank any way you like them!
Heavily fruit flavoured liqueurs are great to enjoy in autumn and winter as they are reminiscent of the autumn harvest of fruits such as plums and damsons.
Damson / Sloe Gin's would generally also fall within the Liqueur family.