Joomla makes extensive use of the Model-View-Controller design
When Joomla is started to process a request from a user, such as
a GET for a particular page, or a POST containing form data, one of
the first things that Joomla does is to analyse the URL to
determine which component will be responsible for processing the
request, and hand control over to that component.
If the component has been designed according to the MVC pattern,
it will pass control to the controller. The controller is
responsible for analysing the request and determining which
model(s) will be needed to satisfy the request, and which view
should be used to return the results back to the user.
The model encapsulates the data used by the component. In most
cases this data will come from a database, either the Joomla
database, or some external database, but it is also possible for
the model to obtain data from other sources, such as via a web
services API running on another server. The model is also
responsible for updating the database where appropriate. The
purpose of the model is to isolate the controller and view from the
details of how data is obtained or amended.
The view is responsible for generating the output that gets sent
to the browser by the component. It calls on the model for any
information it needs and formats it appropriately. For example, a
list of data items pulled from the model could be wrapped into an
HTML table by the view.