Transmission & Gears
Bikes are often defined by the number of gears or 'speeds' that they have, with a 20-speed bike assumed better than a mere 18-speed. That may often be so, but more important than the number of gears is their size. The fastest Olympic cycling events are after all, undertaken on bikes with one single fixed gear! But for most purposes it helps to be able to vary the gear, and for that purpose a bike may have derailleur gears or hub gears - or even both.
It all starts with cranks and pedals (or handles) to push them round, then something: a chain a belt, maybe a shaft, to connect that motion to the driven wheel. The earliest pedal cycles didn't have anything in-between: the cranks were attached directly to the wheel like on a childs pedal toy. Except they were not toys. That wheel was big and got bigger as bikes developed, for that was the only way to cover more ground with each turn of the pedals. This is how the bike we now call a penny farthing evolved and these bikes were rated in inches, by the size of their wheel.
In their day these high wheelers were called ordinary bicycles, to distinguish them from some new thing called a safety bike. This had a chain drive to the rear wheel (sounds familiar?) that could 'gear up' the transmission by having more teeth at the cranks than the wheel. Three times as many teeth on the cranks, a 48 tooth chainwheel and 16 tooth rear sprocket for example, makes a 26in wheel equivalent to a 78 inch ordinary. It would be some tall guy who could straddle a wheel that size!