Thermofoil. These doors are molded out of MDF (medium-density fiberboard), wrapped in a plastic-type coating and then baked under intense heat to create an impervious seal. Durable and cost effective, they come only in solid colors and imitation wood grain.
Often mass-produced, thermofoil cabinetry comes at very competitive price points. It's durable, but it's also extremely difficult to repair any damage to it. Some lighter colors can also yellow from sunlight and heat over time.