Bike frames are the portion of bikes that are not the wheels, pedals, handlebars, or seat. In short, a bike frame is the main support structure of the bike, located primarily between the two wheels and the pedals and the seat. Bike frames can be purchased separately or, more commonly, come included as an entire package.
There are a number of components to bike frames. The first is the top tube, which is the horizontal bar that runs up toward the top of the bike frame. The head tube holds the handlebars and goes down to the fork at the front of the bike. The fork is attached to the wheel. The down tube is a diagonal bar that usually goes diagonally down to the chain wheel from just below where the top tube connects to the head tube. The seat tube runs from the saddle link down to near the chain wheel.
In addition to these main tubes, there are other components to bike frames called seat stays and chain stays. The seat stay goes from the top of the seat tube down to the rear portion of the bike, near where the rear wheel connects to the frame. The chain stay also connects to that portion of the frame, but instead of running to the seat, runs to the chain wheel.
It is important to note that while the above components describe the vast majority of bikes, some bike frames may not have all of these components. The components are often based on the latest available technology, or at least the price one may be willing to pay for that technology. Overall, despite advances in technology, bike frames have changed very little since the "high wheeler" bicycles of the 19th Century.
Bike frames are also made out of a number of different materials. These materials have changed substantially over time to make bike frames lighter, more durable, and provide an addition level of comfort, in some situations. Common materials for bicycle frames include: aluminum, steel, carbon fiber and titanium.
Each of these materials used in bicycle frames may have their own advantages, but for the recreational user it may not matter much. Those who need a performance bike frame will spend a considerable amount studying different materials. Titanium is becoming increasingly popular for performance racing bikes. However, those looking for just a serviceable bike to get around on will likely choose steel, which is one of the cheaper material options.