A fixed gear bicycle, often called a "fixie," is a type of bike that has a single gear and no freewheel mechanism. This means that the pedals are always in motion when the bike is moving, allowing the rider to control speed and braking by pedaling backward. Fixed gear bikes are popular for their simplicity and lightweight design. These bicycles are commonly used in urban environments and by cycling enthusiasts for commuting and racing. They require less maintenance than multi-gear bikes and offer a unique riding experience. However, they can be challenging for beginners due to the lack of coasting ability.