Thermoforming is a process in which plastic sheets are heated until they become pliable, then the plastic sheets are formed into the desired shapes in molds, and trimmed to achieve usable parts. This is a very generic definition of course. There are many types of thermoforming processes, and therefore many types of thermoformers, or thermoforming machines if you prefer. My experience is mostly in producing PS, PP, and PET trays for food products.
In the picture above you can see a thermoformer built to produce PS foam trays. This particular thermoforming machine, and modified versions for PP and PET are built by Sunwell Global, one of the companies that I have more experience with. There are many others though, and not all markets call for the bigger machines.
Commodore Technology for instance build the same type thermoformers, but are better equipped for the smaller and quick changeovers form product to product.
Another thermoformer that is popular for PET and PP parts are built by Inpak Makina.