Thermographic printing refers to two types of printing, both of which rely on heat to create the letters or images on a sheet of paper.
The simplest type of thermography is where the paper has been coated with a material that changes colour on heating. This is called thermal printing and was used in older model fax machines and is used in most shop till receipt printers. This is called direct thermal.
More complex is thermal transfer printing that melts print off a ribbon and onto the sheet of paper.