From Hors d'oeuvre, appetizers are if there is a long period of time between when your guests arrive and when the main meal is served. An example would be during a cocktail hour. Appetizers might also serve the purpose of keeping guests happy (not starving!) during the long wait. In some cases, Hors d'oeuvres are served with no meal afterward. Cases where you might see this are at many reception and cocktail party events.
Hors d'oeuvre may be served at the dinner table as a part main meal or they may be served before sitting at the table. Hors d'oeuvres served prior to a main meal are either stationary on side tables or passed around by the host or workers at those hosts reception location. Stationary hors d'oeuvres are also referred to as table hors d'oeuvres.