Custard is based on a cooked mixture of milk or cream and egg yolk. It may vary in consistency from a thin pouring sauce to a thick pastry cream. Most common custards are used as desserts or dessert sauces. Custard preparation is by all means a delicate operation; a temperature increase of 3 to 6 °C can already lead to overcooking and curdling. Generally, a fully cooked custard should not exceed 80 °C; it begins setting at 70 °C.
Reference: Wikipedia