The plural form is used in English because it refers all of the individual appetizers. It would not be incorrect to refer to a single unit as an hors d'oeuvre. In French, the spelling hors d'œuvre is both singular and plural.
hors· d'oeuvre (pronounced or-DURV) is a noun. A small portion of a tasty food served as an appetizer before a meal or as at a cocktail party. From the French, and literally means "outside of work"...
You are probably refering to "crudités" and "charcuterie"; the first being vegetable preparations and the latter being meat based, usually porc (pâté, salami etc...)
Just as a bit of useless info...