They are generally single-use, disposable items made of pleated wax-treated paper or pleated foil. You can also get reusable ones made from silicone. They are sold stacked in a box. The foil ones are stacked separated by paper muffin cups. The package directions often say to discard these paper ones, but many people use them anyway. Household-size boxes contain 25 or 50 of them. Commercially, of course, cases will often hold up to around 10,000 of them.
Baking cups come in different sizes to fit different sized muffin tins. At home, to determine the size of a baking cup, flatten the cup out, and measure through the middle from side to side (you probably won’t ever need to do this if you store them in the box they came in, which will give the size.) Commercially, to be more accurate when ordering tens of thousands of these at a time, measurements will be given in overall width as above, but the measurements will also include the height of a side wall, and the width of the centre base.
The design on baking cups can be plain or quite decorative. Foil ones can be regular “silver” foil colour, or gold, or other colours. Both foil and paper ones are often holiday themed, particularly at Christmas and Hallowe’en.
Paper ones are better to use if you are planning to reheat muffins in a microwave. Some foil ones are strong enough to stand up on their own; you don’t need a muffin pan. They can be an ideal choice when you need to make a lot of muffins at once but only have 1 or 2 muffin pans. If you plan to freeze the cupcakes or fairy cakes, use the foil liners. Paper ones will get soggy when you go to thaw them.
Silicone baking cups are also strong enough to stand up on their own, and are re-usable indefinitely. They are oven-safe, microwave-safe, and can also be used in pressure cookers to make small savoury dishes such as egg bites, etc. Check with the manufacturer of yours about freezing in them.
Paper baking cups can be composted after use; foil ones may be accepted in recycling, depending on where you live.
Light patterns on a white background might not show up well if your batter is also a light coloured one.
Baking cups are also used for making caramel apples in (for standing the apples up in them.)
You can get baking cups that are certified kosher.
Fill baking cups only two-thirds full with batter to allow for rise of the batter during cooking.
Sometimes referred to as “bake cups”, and popularly also referred to as “muffin cups.”