It is a series of metal cups connected by a flat metal surface. The cups connect to the metal surface at the lip of each cup.
Jumbo-sized muffin tins will make 6 muffins. Standard muffin tins will make 12 muffins, with each cup in them holding between 1/3 and 1/2 cup (2.5 to 4 oz / 75 ml to 125 ml) of batter each. Mini muffin tins will hold between 1/8 and 1/4 cup (1 to 2 oz / 30 to 60 ml) each of batter and will make 24 muffins.
The lightweight muffin tins made out of standard aluminum seem to do as good a job at even cooking and browning as the more expensive, heavier ones do. You can get non-stick muffin tins, which you should still grease, or use muffin tin liners (also called cupcake liners or, more neutrally, baking cups.)
Only fill the cups two-thirds full. Always wipe off any spills on the flat surface between cups before baking. It’s far more work to clean after it’s all burnt on.
They are also good for baking stuffed tomatoes and peppers in. Stuffed tomatoes and peppers sometimes collapse outward as they cook, but baking them in muffin tins will support them. Grease the tins first.
If what you are making doesn’t require all the cups in a muffin tin, some people advise to fill the empty ones with water to prevent them from smoking in the oven.
For small muffin cups, an alternative name is tassie pan, used in the American South. In this parlance, the individual cups in the pan are called “tassie cups”.