The two types of custards are similar, but they do have some differences. Flan is generally baked in deeper ramekins. The custard is a bit less rich (not necessarily made with heavy cream but with a mixture of cream and milk, or half and half). The custard mixture for flan gets poured into ramekins lined with caramelized sugar. This sugar melts during baking and refrigeration, and when you invert the flan to serve, the caramel serves as a sauce. Creme brulee is served in its ramekin and flan is served turned out onto a plate. Flan is served chilled through while creme brulee is served cold on the bottom and warm on top.