Waterfall approach was first SDLC Model to be used widely in Software Engineering. In “The Waterfall” approach, the whole process of software development is divided into separate phases. In Waterfall model, typically, the outcome of one phase acts as the input for the next phase sequentially.
The sequential phases in Waterfall model are:
- Requirement Gathering and analysis: All possible requirements of the system to be developed are captured in this phase and documented in a requirement specification doc.
- System Design: The requirement specifications from first phase are studied in this phase and system design is prepared. System Design helps in specifying hardware and system requirements and also helps in defining overall system architecture.
- Implementation: With inputs from system design, the system is first developed in small programs called units, which are integrated in the next phase.
- Integration and Testing: All the units developed in the implementation phase are integrated into a system after testing of each unit.
- Deployment of system: Once the functional and non functional testing is done, the product is deployed in the customer environment or released into the market.
- Maintenance: There are some issues which come up in the client environment. To fix those issues patches are released. Also to enhance the product some better versions are released. Maintenance is done to deliver these changes in the customer environment.
Advantage
The advantage of waterfall development is that it allows for departmentalization and control. A schedule can be set with deadlines for each stage of development and a product can proceed through the development process model phases one by one. It’s simple and easy to use.
Disadvantages
No working software is produced until late during the life cycle, and it isn’t a good model for complex and object-oriented projects, because all team is working on each part 
More information read here:
http://www.tutorialspoint.com/sdlc/sdlc_waterfall_model.htm
