The Cover is the outer-most layer of a golf ball. In premium golf balls including the Titleist Pro V1®, Pro V1x®, and Pro V1x Left Dash, the cover is Urethane Elastomer - a soft yet tough, synthetic polymeric material. There’s also Thermoplastic urethane or TPU which utilizes pelletized urethane that is injection-molded to form the outer cover. This is the cover you’ll find on the Titleist AVX® golf ball.
Other golf ball models typically feature blends of ionomers (Titleist models include Velocity™, Tour Soft and TruFeel™). Ionomers are harder, more durable plastics like Surlyn®. Ionomer covers are more durable, however, the advantages of a urethane cover compared to ionomer include soft feel, increased spin in the short game, and an improved ability to control shot shape and distance.