Wurm is a creature type.
It is the term for a type of creature that acts as a Dragon of sorts for Green, although other colors have one or two wurms too. Mostly, they are giant snake or worm-like creatures. They have high Power and Toughness and they tend to cost a lot of mana, but can be very effective. Some of them have effects like Trample, but others just have high Power/Toughness.