A hybrid is a specific class of bicycle that combines the features of a road bike and a mountain bike into a commuter-friendly ride suited for a little of both terrains. Hybrids, which can also be called "cross bikes" or "fitness bicycles," borrow the large wheels from road bikes and the higher handlebars and heavier frame from mountain bikes. This makes them faster and more aerodynamic than a mountain bike but sturdier and slower than a traditional touring bicycle. Specific modifications such as high handlebars, chain guards and fenders make the hybrid an attractive option for bike commuters.
Hybrid bike frames split the difference between mountain bikes and road bikes. The heavy design makes the hybrid frame sturdier than a road bike. Another holdover from the mountain bike frame, the high bottom bracket, makes it easier to go over obstacles in your path, such as tree branches or curbs. However, according to "Sheldon Brown's Bicycle Dictionary," the persistence of the high bottom bracket in hybrid bikes may pose problems for in-town cyclists, who often need to dismount at stoplights.
Wheels and Tires
Hybrid bikes tend to have larger road bike-sized wheels but with wider tires. Classic hybrid tires are intended to make the bicycle more versatile, maintaining a smaller -- and thus faster -- physique, while adding some traction for off-roading. Many hybrids come with "knobbie" tires that are noisy and sluggish on hard surfaces. They are designed for sand or mud, but the hybrid version of these tires is too narrow for this purpose. The size of the actual wheels has relatively little effect on the speed of the bicycle.
The Ugly Middle Ground
Riders of hybrid bikes will find that they are slower on pavement than conventional road or touring bikes. Hybrids might also be found lacking in the traction department when riding uphill or traversing uneven or slick surfaces. Ultimately, if you are entering bike races or roughing under-maintained trails, you may want to go for a bicycle specifically designed for your activity.
The Draw for Bike Commuters
Hybrid bikes are often made with features that are helpful for bicycle commuters. Fenders, or rain guards, keep rain and mud off the rider's clothes, and chain guards protect slacks or jeans from being torn in the gears. Handlebars higher than those featured on a mountain bike help the rider be more visible in traffic.