Hors d'oeuvres (pronounced "or-DERVS") are small one- or two-bite items that are served before a dinner, usually accompanied by cocktails, or in place of dinner at a cocktail party where a full dinner is not being served.
Hors d'oeuvres can be served at a table or passed on trays among guests. Note that the word appetizer is sometimes used interchangeably with the word hors d'oeuvres, which can lead to confusion in cases where the word appetizer is understood to mean the first course of a meal.
There's no hard and fast rule, but in general, an hors d'oeuvre is served before a meal, and an appetizer is part of a meal.
Note that technically the word hors d'oeuvre, without an 's' at the end, can be used to indicate the singular and also the plural form of the word. But in America, at least, it's normal to say hors d'oeuvres (plural) to mean more than one.
Types of Hors d'Oeuvres
There are a few general categories of hors d'oeuvres. Canapés are a type of hors d'oeuvre constructed of a base of made of bread, pastry, crackers or something similar, with some sort of spread such as flavored cream cheese or butter, and then a topping. The topping can be practically any sort of meat, seafood, vegetable, fruit and so on.
Another sort of hors d'oeuvres would be small items served on a skewer, stick or toothpick such as grilled shrimp, satay beef skewers or cheese and fruit kebabs. Small pastry shells filled with some savory or sweet ingredient would also be a type of hors d'oeuvre.
Crudité platters or even dips served with crackers or chips can be considered hors d'oeuvres as well. Indeed, a simple bowl of nuts could be considered an hors d'oeurve if served with drinks before dinner.
There's really no limit to what sort of item can be considered an hors d'oeuvre, provided it is small (one to two bites) and is served before a dinner or in place of dinner at a cocktail party.