The images below are intended to provide a simple representation of the difference between sequential and iterative development approaches. They are taken from ‘Changing Times: Quality for Humans in a Digital Age’.
Sequential models such as Waterfall or V-Model rely on intensive periods of collecting and refining requirements for a product before design and development activity can take place. Products developed using these models are intended to be complete when released to customers. Central to the approach is an assumption that by adhering to the requirements captured at the outset, the product will fulfil the wishes of those customers:
This is in contrast with iterative ways of working, with an emphasis on delivering less complex products, often to test customer response before making adjustments. The techniques encourage regular feedback from customers, and rapid response to that feedback; refining ideas and revisiting design and development activities with the intention of delivering products which better reflect what customers want: