Some are somewhat mechanized, with a handle that you turn or squeeze. Some have interchangeable discs that have differently-sized holes.
Some are completely manual, like a sieve in an inverted cone shape, and you work a pestle (usually made from wood) round and round in them to force the food item through. These are called rotary ricers.
Most are made of stainless-steel, though some are made out of aluminum. The stainless-steel ones are more likely to be dishwasher-safe, but check before you buy.
You can purée apples through one, or use it instead of a potato masher to make mashed potato from. It doesn’t cause the starch in foods like potato to become glutinous. You can also use it to press spinach dry.
For potatoes, rub cooked potatoes against a colander.