What is the Waterfall Model?

The waterfall model is a sequential software development model in which development is seen as flowing steadily downwards (like a waterfall) through several phases.
1) Requirement Collections
2) Feasibility Study / Analysis
3) Design
4) Coding
5) Testing
6) Implementation & Maintenance  

Requirement Collections: There are two ways gathering the requirements from  clients to whom your developing the s/w or research in the market for customer needs.
Feasibility Study / Analysis: What are all the possibilities going ahead and complete the task. It can be done in two ways 1) Technical point of view and 2) Business point of view.
Design: 
i) HLD (High Level Design): What are all modules we have, how the communication takes place between one modules to other module, what type of architecture to follow and what is the front end & back end to be used will comes under High Level Design.
ii)LLD (Low Level Design): Its a small feature in the high level module.
Coding: coding is done to the lower level design. coding will be done using programming languages.
Testing: Once coding is done by the developers. Testing team will verify whether it is working according to the requirements.