Appetizer is the comprehensive term denoting food or drink served in advance of a meal as a whet to the appetite; it may include cocktails and a savory tidbit served before going to the table or a first course (as of oysters, clams, fruit cocktail, or canapé) served at the table.
Hors d’oeuvre may be used of a savory, salt, smoked, tart, or uncooked food served with cocktails or as a first course at table. Its more common plural form, hors d’oeuvres, generally suggests a tray of such foods from which one selects what pleases one’s taste.
An aperitif is a small drink of liquor and especially of a fortified and usually specially flavored wine taken shortly before lunch or dinner for the purpose of or under the pretext of stimulating the appetite.