This is commonly known as an ensemble or groupset, and includes the cranks, chain, sprockets, hubs, bottom bracket, brakes and gear levers. The quality, and price, increases as you progress through the classifications, from entry level to elite. Going up in levels will give increased performance, for example the levers will change gears more fluently and easily and will be lighter in weight with greater strength.
Generally when buying a bike, and once a frame is selected, experienced buyers will go for the highest level of componentry that their budget will allow.