They are usually savory, but can be sweet. They are best eaten hot right away.
You make fritters either from a dough or a batter with chopped food in it, or take food items, and coat them in a batter.
There is always a starch involved in the dough or batter, and usually, some leavener.
There is usually one main ingredient besides the batter, though to most people’s hungry eyes that can appear to be the whole point of them. But in a good fritter, the main ingredient’s taste isn’t overwhelmed by the batter.
Food items inside a fritter may or may not need cooking on their own first, depending on the food item.
Corn fritters and apple fritters are popular in America; mushy peas in batter is a popular fritter in Britain. Indian food has pakora, which are fritters.
Fritters have been popular since Roman times (at least) up until today. The Romans made “scriblita.”Spoonfuls of leavened dough were placed in a pot of hot fat.” 
Medieval sweet fritters would contain wine or ale.
“Fritter” comes from the Old French word for frying, “friture,” which in turn came from the Latin “frictus”, past participle of “frigere.” (“Frigere”, confusingly, can also mean at times “get cold.”) From the same root comes the Italian omelette-like dish called “frittata.”
 Davidson, Alan. The Oxford Companion to Food. Oxford, England: Oxford University Press:Oxford. 1999. Pages 320-321.