Basic Information
A moat is a type of fortification consisting of a large ditch. The term is slightly archaic and would probably not be used in the modern era, even if it describes such things as anti-tank ditches precisely. Moats are traditionally placed in front of other defences (such as walls) both as an additional obstacle and to impede the reduction of those defences.
The simplest form of moat is nothing more than a deep dirt sided ditch. More advanced forms may be lined and/or flooded - if nothing else the lining allows the sides to be cut at a steeper angle, but in a wet ditch it also serves to retain water and prevent erosion. By the early modern period moats were generally dry with vertical wall sides and lay between the glacis and the walls. Often these moats contained firing positions below ground level allowing the defenders to fire along the length of the moat and enfilade attacking troops as they attempted to cross.
More generally, and especially once firearms had been introduced, a moat could generally make adequate cover for additional infantry firing positions.
The bottom of a moat may also be filled with abbatis of one form or another - this is commonest in dry moats, but a combination of spikes and water may be especially vicious, a modern version being the placing of barbed wire or razor wire entanglements below the surface of the water.
In more primitive conditions the moat may serve as a water source and/or a sanitary convenience and/or may be stocked with fish. A moat may also be stocked with more aggressive species if available - pirahna and crocodiles are possible choices - although this was historically uncommon, probably because they were less use and more nuisance than might at first be thought1.
In fantasy works, moats may also be filled with magma, acid and other wierd substances. There are no known historical examples of this2, probably due to the massive logistical difficulties involved in building and maintianing them.