Thermoforming is a manufacturing process to form the plastic sheet into a specific shape via mold at a pliable forming temperature. The plastic sheet or film is heated in an oven to a high-enough temperature that permits it to be stretched into or onto a mold and cooled to a finished shape. Injection molding is a manufacturing process where molten plastic is injected into one or more cavities in a three-dimensional mold. This mold opens and closes as parts are formed and purged from the mold cavity.
Thermoforming is used for smaller production quantities and is more limited in terms of what types of materials can be worked. Injection molding is normally used for large volume production runs with wide selections of plastic as base material.