Your bike's groupset comprises everything that makes it go and stop.
It's important that all these components work together seamlessly. Here's a typical road bike groupset.
The individual components are:
1. The crankset comprises one, two or three chain rings. The chain ring provides the drive to the rear wheel via the chain connecting it to the cassette. The crankset is attached to levers to which the pedals are fastened. The chain rings typically comprises 1, 2 or 3 toothed gears which allow a major gear change. The more teeth, the higher the gear.
2. The rear cassette. The rear cassette has a larger number of cogs (usually 7 to 11) of different sizes and is connected to the rear axle. At any given time, one of the front chain rings is connected to one of the rear cogs via the chain, such that rotating the chain ring using the pedals rotates the rear cog at the same rate.
3. The chain. Connects the front chain ring to the rear cassette. The highest gear is when the largest chain ring is connected to the smallest cassette cog and conversely, the lowest gear is the smallest chain ring to the largest cassette cog.