A software framework is a reusable design for a software system (or subsystem). This is expressed as a set of abstract classes and the way their instances collaborate for a specific type of software. Software frameworks can be object-oriented designs. Although designs don't have to be implemented in an object-oriented language, they usually are. A software framework may include support programs, code libraries, a scripting language, or other software to help develop and glue together the different components of a software project. Various parts of the framework may be exposed through an application programming interface (API).
- From Wikipedia
Joomla! 1.5 is a three tiered system. The bottom tier is the framework level and consists of the libraries and plugins (formerly known as mambots). The second tier is the application level and consists of the JApplication class. Currently there are four applications that ship with Joomla: JInstallation, JAdministrator and JSite. The application acts as the main controller for the page.
The Joomla! Framework consists of several different packages.