CodeIgniter is a PHP MVC framework that allows developers to build applications rapidly. It provides libraries for connecting to databases and performing common operations like sending emails and uploading files. CodeIgniter follows an MVC architecture where the model manages the data, the view displays it, and the controller processes requests and passes data between them. It is lightweight, supports query building and form validation, and includes security features like XSS filtering and password hashing. CodeIgniter is installed by uploading its files and folders to a server and configuring settings for the base URL and database connection.