Each unit consists of a plastic base with a thick felt pad on top. They seem pretty well made and substantial in build quality. I am not sure they would be suitable for a hard floor surface as they might slide around somewhat but to protect a thick and expensive carpet they seem fine.
The price here at just £2.20 for four seems good for the quality of the item and large 70mm base means that even pretty hefty chair legs should fit.
I would be happy to recommend them.