To understand the difference between a production line and an assembly line it helps to look at how each method creates a final product. While each process is similar, there are a few fundamental differences between these two types of manufacture.
In a standard production line, products move from station to station simultaneously. On each station a set of operations is performed. This could include a process such assembling and securing parts of a machine. These operations are carried out successively, and each one adds to the final creation of the product.
Assembly lines however work by adding components to the product at certain stations. These components are then assembled with the main product, components are often pre-fabricated and designed so that they can be added in this way. The main reason why this is done is to improve productivity whilst minimising the cost of mass production.