Metal Basket Steamer
© Denzil Green
A Steamer is a device that has boiling water below and above it, a dish or layers of dishes or baskets that have perforations in their bottoms, allowing steam that rises from the boiling water below to rise up through them, and cook the food in the dish.
It's meant to keep food out of boiling water, so that it cooks by steam, not by boiling. Many foods will keep their flavour, texture, colour and shape better when steamed versus boiling.
A Steamer can be a special unit that comes with both a special pot and the steamer layer above. They can be meant for stove top use, or they can be electric.
It can also be assembled using an insert that goes into or on top of regular pots. They can be made of aluminum, stainless steel, enamelled metal, bamboo, etc.
The inserts are often called "baskets." They fold outwards in metal "leaves" from the centre, allowing them to fit a wide range of pots. Even though they are collapsible for easier storage, they can be a nuisance to store or handle. The "leaves" fall out of place, or trip over each other. They are really not meant to last forever.
Steamers that allow for stacked levels allow you to steam different foods at the same time separately, or to steam more of the same food at the same time without crowding it.
Bamboo steamers can be used over woks. Bamboo flavours can absorb flavours from foods, so you may wish to keep a separate one for fish. For more details on bamboo steamers, see the separate entry on them.
Stainless ones can pick up what looks like rust from acidic foods, though often it will come off after a run through a dishwasher.
The boiling water shouldn't touch the steamer layer. 2 inches (5 cm) of water should be enough to steam for up to 45 minutes. There are exceptions -- tall steamers specially for asparagus are designed for the asparagus stalks to be standing up, with their tougher base in the water to cook the bases more.
Covering a steamer will help to keep most of the moisture in the pot, so that it doesn't boil dry. If you're steaming for long periods, you should still check on the water level, even if you have the steamer covered.