A production line is a set of sequential operations established in a factory whereby materials are put through a refining process to produce an end-product that is suitable for onward consumption; or components are assembled to make a finished article.
Typically, raw materials such as metal ores or agricultural products such as foodstuffs or textile source plants (cotton, flax) require a sequence of treatments to render them useful. For metal, the processes include crushing, smelting and further refining. For plants, the useful material has to be separated from husks or contaminants and then treated for onward sale.
Early production processes were constrained by the availability of a source of energy, with wind mills and water mills providing power for the crude heavy processes and manpower being used for activities requiring more precision. In earlier centuries, with raw materials, power and people often being in different locations, production was distributed across a number of sites. The concentration of numbers of people in manufactories, and later the factory as exemplified by the cotton mills of Richard Arkwright, started the move towards co-locating individual processes.
Introduction of the steam engine 
With the development of the steam engine in the latter half of the 18th century, the production elements became less reliant on the location of the power source, and so the processing of goods moved to either the source of the materials or the location of people to perform the tasks. Separate processes for different treatment stages were brought into the same building, and the various stages of refining or manufacture were combined.
Industrial revolution 
With increasing use of steam power, and increasing use of machinery to supplant the use of people, the integrated use of techniques in production lines spurred the industrial revolutions of Europe and the United States.
Assembly line 
Thus, from the processing of raw materials into useful goods, the next step was the concept of the assembly line, as introduced by Eli Whitney. This was taken to the next stage at the Ford Motor Company in 1913, where Henry Ford introduced the innovation of continuously moving the cars being assembled past individual work stations. This introduced the idea of standardization.
See also 
|Wikimedia Commons has media related to: Production line|
- production line - examples