Bike frames are not the foundation of a bike; that honor goes to the wheels. However a frame is the biggest portion of the bike. To use the analogy of a body rather than a house; if the wheels are the feet, the frame is the torso. The legs… well you have to supply the legs for the bike yourself.
Bike frames have come a long way from the wooden frames that accompanied the first models. The first frames didn’t follow the geometry that we have now, but rather just looked to hold the wheels while allowing them to spin. The seat was wooden, and not padded, and there was no attachment point for pedals anywhere to be found.
Eventually bike frames became standardized around a diamond formation. The diamond consists of a top bar, which is the longest piece on most bikes. Since the diamond shape needs four sides, we add a front post, seat post, and bottom post to the mix to complete it. The composition of the frame has also shifted, from wood to steel, from steel to aluminum, from aluminum to carbon fiber and Kevlar. Today you can find bicycle frames in all four of the latter compositions, at prices ranging from under 100 for a cheap steel frame, to over 10,000 for a good carbon fiber or Kevlar frame.