Explanation:

Waterfall Model:
The waterfall model is a sequential software development process in which progress flows steadily downwards, like a waterfall, through the phases of requirements analysis, design, coding, testing, and maintenance. Each phase must be completed before the next phase can begin.

Development Phases of the Waterfall Model:
1. Requirements Analysis and Specification: In this phase, the requirements for the software system are gathered and analyzed. The requirements are documented in detail, including functional and non-functional requirements, user expectations, and system constraints.

2. Design: In this phase, the system design is created based on the requirements. The design includes the overall architecture of the system, the database design, user interface design, and other design elements. The design phase serves as a blueprint for the development team.

3. Coding and Unit Testing: In this phase, the software is developed based on the design specifications. The code is written, compiled, and tested in small units called modules. Unit testing is performed to ensure that each module functions correctly and meets the requirements.

Not a Development Phase:
4. Maintenance: Maintenance is not a development phase of the classical waterfall model. It is a separate phase that comes after the development is complete. Maintenance involves making changes and enhancements to the software to fix bugs, improve performance, and add new features based on user feedback and evolving requirements.

Importance of Maintenance:
- Maintenance is crucial as software systems need to evolve and adapt to changing user needs and technological advancements.
- It ensures that the software remains usable, reliable, and secure over time.
- Maintenance also includes activities such as documentation updates, troubleshooting, and providing customer support.

Conclusion:
In the classical waterfall model, the development phases include requirements analysis and specification, design, and coding and unit testing. Maintenance is a separate phase that occurs after the development is complete.