The moulds come in various sizes and are made from various materials.
Oval ones with tall sides are made of aluminum; you improvise covers with tin foil. Stainless steel ones tend to be round, and have lids that you latch on.
Some flan moulds are actually combined mould / steamer sets. They consist of a double-boiler, with the bottom pot holding the water, the top pot being the flan mould, with a tube in it (reminiscent of an angel-food cake pan) that allows the steam to rise up into the centre and cook the flan more evenly and quickly.
If you are using your flan mould in a water bath, the water should come up about ⅓ of the way up the sides of the mould.