A mousse is a light fluffy mixture, either sweet or savoury; it can be served hot or cold. There are no hard and fast rules, but sweet mousses are often flavoured with chocolate or fruit purée and many contain whipped cream.
Savoury mousses usually contain puréed or blended fish or meats such as salmon, shellfish, chicken or ham. Sweet mousses tend to be made with beaten egg whites while savoury mousses use gelatine.
Some mousses just need setting in the fridge while other recipes might call for them to be baked in the oven. Use pasteurised eggs if making a mousse in which the egg is not thoroughly cooked.