CodeIgniter is a PHP framework for web application development. It provides a lightweight yet powerful platform for building web applications, especially for developers looking for a fast and uncomplicated development environment. CodeIgniter is known for its simplicity, flexibility, and speed, and it has an active developer community continuously working on its improvement.

Some of the key features and advantages of CodeIgniter include:

1. Lightweight structure: CodeIgniter is relatively slim and does not require many system resources, making it quick to install and easy to use.

2. MVC architecture: The framework follows the Model-View-Controller (MVC) pattern, allowing for the separation of business logic, presentation layer, and database access.

3. Easy configuration: CodeIgniter offers a simple and clear configuration, allowing developers to get started quickly.

4. Database support: The framework supports various databases, including MySQL, PostgreSQL, SQLite, and others, and provides an easy interface for querying and manipulating data.

5. Security: CodeIgniter includes features to enhance the security of web applications, such as input data validation and protection against Cross-Site Request Forgery (CSRF).

6. Libraries and helpers: CodeIgniter includes a variety of libraries and helpers that facilitate common tasks like form validation, session management, and image processing.

7. Community support: The framework has an active and dedicated developer community that provides numerous resources, extensions, and training materials.

CodeIgniter is particularly well-suited for small to medium-sized web projects where a simple yet powerful solution is needed. It offers the flexibility to meet custom requirements and allows for the integration of third-party libraries and components.