A soufflé is a savory or sweet cake made from egg yolks and beaten egg whites. The term soufflé comes from the French verb souffler which means "to puff up": essentially describing what happens to the combination of egg and other ingredients. The base ingredients for a souffle are a flavored cream sauce or puree, and egg whites beaten to form a soft peaks as a meringue.
Soufflés should come out of the oven fluffy and puffed up. It will generally fall after about 5 to 10 minutes afterwards.