Fat Bikes are bikes with a wide tire, typically between 4.5-5". This allows for a much lower pressure to be run in the tires, providing incredible traction and a smooth, comfortable ride. This also allows for these bikes to be ridden in the snow!
For an awesome alpine experience, be sure to visit one of the local ski resorts for a day of fat biking. There are rental fleets located at Apex Mountain Resort, and Baldy Mountain Resort for the winter.