They are made of nylon resin or mylar, by at least two companies (as of 2011): Reynolds (who calls them “Slow Cooker Liners”, and Regency Wraps, who calls them “Slow Cooker Savers.”
Generally, the liners are made of the same material as “plastic” oven bags, but are a different size and shape and have different openings. Unlike oven bags, they have no ties, as they are meant be be open at the top of the slow cooker.
The Reynolds-made liners are 13″ x 21″, designed to fit 3 to 6.5 quart round or oval slow cookers, and come four in a box. They are also certified Kosher.
Slow Cooker Liners are not for oven, broiler, toaster oven, or barbeque grill use.
Don’t use them to store food inside them in the fridge, either. You can however line a slow cooker crock with one, put the food in it, cover and refrigerate, then cook later.
Don’t lift the liner up with food inside it; serve food directly from the lined slow cooker crock. When the slow cooker has completely cooled, use a spoon, etc, to remove any remaining food, then remove the liner and discard.
Some people complain that they buy the liners, and forget to use them — a tip from fans of the liners is to try storing the box of liners inside the slow cooker, where you’re bound to spot it when you reach for the slow cooker.
As with any product, there are fans and detractors. Some say that you don’t need them: that simply soaking the crock in hot soapy water, sometimes with a handful or two of baking soda in it, easily solves any cleanup problems. Some say that spraying the inside of the crock with cooking spray first before putting food in will do the same job. Still others say they just don’t like the idea of adding to landfill with yet more disposables.
Others say that it’s just as environmentally unfriendly to be running all that hot water and using soap and other chemicals to soak and scrub a crock, and that those costs, combined with the time saved, easily make the product idea a winner.
Slow Cooker liners are particularly popular with the motorhome community, because of the cleanup hassle in limited space (with limited hot water) that the liners can save.
Note that if you are using a Slow Cooker Liner, you don’t need to spray the inside of the crock with any cooking spray first; it would just be a waste.