The first cars were built largely by coach builders. Early automakers would buy engines from a manufacturer and install them in a modified horse coach. In fact, that's why even today companies that hand-build cars are sometimes referred to as coach builders. For the most part, those builders employed skilled craftspeople to make their coaches custom tailored to meet each order. Buyers could choose exactly what they wanted their new automobile to look like -- inside and out.
However, coach builders soon discovered they could build more cars faster if they standardized the design and parts. Rather than fabricating every part in each car, the vehicle's components could all be made using molds and machines. The workers would then simply assemble the finished product.
While many people think that Henry Ford invented the automotive assembly line, it was actually invented by Ransom Eli Olds. Olds had worked on cars for most of his life, including steam-powered cars in the 1880s and 1890s. His assembly line allowed him to be the first mass-producer of cars in the United States, and he dominated the American automotive industry from 1901 to 1904.
However, the reason most people think Henry Ford invented the automotive production line is because Ford took the idea and improved it. Ford's assembly line was actually based on a slaughterhouse's de-assembly line. While Olds' assembly line for cars may have been the first, Henry Ford's assembly line was built on essentially the same idea and was far more efficient. Ford's line assigned workers to one specific production task. Each task had a production station. A car would arrive at the station, and the worker would perform the specified task -- over and over again on each car that came by. Because each worker had one task and worked on just one car at a time, it meant that hundreds of cars were being built simultaneously throughout the factory. At Ford's original factory, a Ford Model T could be assembled in 93 minutes from start to finish. In fact, every three minutes, a completed car rolled off the production line.
Having the product come to the worker and assigning the worker to perform the same task on each car made production much more efficient and brought the cost of producing the cars down, too. This brought new vehicle prices down considerably and put cars into the hands of people that formerly couldn't afford such a luxury.