These are the most widely used laminators: they use heat to seal the document (or whatever) between two sheets of laminating film. This it is more correct to refer to "hot" or "thermal" roll laminators.
The film has a clear polyester (or nylon, or other material depending on the application) base and an adhesive on one side. Two rolls of film are mounted, one above and one below the laminating device. They are heated by the machine until such time as the adhesive is melted: the rollers than start pulling the film forward to a point where the two films are closest together — the "nip" point - at which point the item to be laminated is inserted into the machine. A second set of rollers pull the whole package through the nip and out the back. The film is kept at the correct temperature throughout the process by a pair of heat-emitting elements called shoes (although some machines have heated rollers instead).
That's the entire basic process. Depending on the machine and the volume of material to be laminated, there are many variations. Controls for speed of the films through the device or the laminating temperature are common. High-volume machines have automatic feeders and may have automatic cutters (on small, low-volume machines the process has to be started and stopped by hand and the laminated item has to be cut off the film and trimmed by hand).
Choosing correct film may be a matter of concern, depending on the application. For most home or small business use one sort of film, requiring a particular temperature range, will suffice. The film material and its thickness will dictate this: rather counter-intuitively, the thicker the material, the lower the laminating temperature will be. For all applications, your supplier shall be able to help you with all the details.