Most camping mattresses are made from foam. The thinner pads that don't self-inflate are made from closed-cell foam, which is super dense.
The self-inflating mattresses are made with less dense foam that allows the open areas in the foam to fill up with air (and also to deflate easily and be rolled up tightly).
The simplest and least expensive camping mattresses will be 100 percent made of this foam, but more expensive ones may offer additional padding for comfort, waterproof covers, etc.