In direct thermal printing the printhead applies heat directly onto a heat-sensitive material, which changes colour to display the data required. In thermal transfer printing, the printhead heats a wax or resin ribbon against the label material instead.
While both printing methods have their benefits, thermal transfer printing is by far the more durable option. This is especially true when labels are exposed to chemicals, solvents and high temperatures (including sunlight), as these may destroy data printed on direct thermal labels.