A hybrid is exactly what it sounds like – a mix of both a road bike and a mountain bike. The result is a bicycle that's suitable for general-purpose riding over various types of terrain, which is why most hybrids tend to be used for commuting.
Hybrids usually have a flat handlebar, as found on a mountain bike, rather than the drop bar you'd find on a road bike. This means shifting and braking components are often very similar to mountain biking units, instead of being STI-style integrated brake and shift levers as you'll find on nearly all road bikes.
This allows for a more upright riding position – not as much outright speed but a position that many will find more comfortable, and potentially safer, in traffic. Hybrid bikes also tend to use larger volume tyres than road bikes, offering more comfort than narrower road rubber.
Some hybrids are closer to mountain bikes than road machines, and others fall closer to the tarmac side of things. Wheel size is usually a good indicator, hybrids using a 26in format commonly found on mountain bikes tend to be more off-road biased. Bikes with 700c road style wheels are usually less ideal for off-road use.
Braking can vary from model to model, but V-brakes are very common nowadays, as are mechanical and hydraulic disc systems. Hybrids usually offer versatile gearing; triple chainsets and wide-ranging cassettes are typical. There are exceptions, though – hybrids can sometimes get hub gears for near-maintenance-free mile-munching, while singlespeed models are also available if simplicity is your thing.
Some hybrids come with full mudguards, and even pannier racks, while most are made with the eyelets and clearances required to retrofit such items.
The Specialized Crosstrail is an example of a hybrid
Some hybrid bikes will also be fitted with a suspension fork. These are usually basic units that will provide comfort for bumpy terrain but offer very limited use for off-road riding.
The important thing to remember with a hybrid bike is that it's a Jack of all trades yet a master of none. A road bike will be faster, a mountain bike will be much better off-road. But for someone who dips in and out of disciplines or who wants to ride mixed terrain in comfort, it can be ideal.