In the legacy Waterfall model, software is developed in the sequential model. On completion of the requirements set, the process moves into design, followed by coding. When we enter the design phase, any change to the requirements is not allowed. As the name suggests, primarily, the water only flows down, you can't make it go up. This is depicted in the following diagram:

Scrum is the popular term used in the Agile model. Actually, Scrum is one of the software development models in the IT industry to support the emerging industry. Nowadays, IT expects the faster and better deliverables in shorter cycle to support the cost efficiency. In alignment with this strategy, Scrum is designed with a few small teams of intensive, inter-dependent, self-sustaining characteristics. In fact, the deliverables are divided into a small intermediate level, named as Sprint.