http://en.wikipedia.org/wiki/Appetizer
Appetizer has several meanings:
Appetizer is another name for hors d'�uvre, a small course served right at the beginning of a meal, before the soup or entr�e.
http://en.wikipedia.org/wiki/Hors_d%27%C5%93uv re
The purpose of the hors d'�uvre is to whet the appetite; if there is a long waiting period between when the guests arrive and when the meal is served (for example, during a cocktail hour), these might also serve the purpose of sustaining guests during the long wait. Hors d'�uvres are sometimes served with no meal served afterward. This is the case with many reception and cocktail party events.
Hors d'�uvre may be served at the table; for example, as a part of the sit-down meal; or they may be served before sitting at the table.
A more substantial starter or first course served at the table might be referred to as an entr�e (outside North America).