As an architectural pattern
It is common to split an application into separate layers that run on different computers: presentation (UI), domain logic, and data access. In MVC the presentation layer is further separated into view and controller.
MVC is often seen in web applications, where the view is the actual HTML page, and the controller is the code that gathers dynamic data and generates the content within the HTML. Finally, the model is represented by the actual content, usually stored in a database or in XML nodes, and the business rules that transform that content based on user actions.
Though MVC comes in different flavors, control flow generally works as follows:
- The user interacts with the user interface in some way (e.g. presses a button).
- A controller handles the input event from the user interface, often via a registered handler or callback.
- The controller notifies the model of the user action, possibly resulting in a change in the model's state. (e.g. controller updates user's Shopping cart).[3]
- A view uses the model (indirectly) to generate an appropriate user interface (e.g. the view produces a screen listing the shopping cart contents). The view gets its own data from the model. The model has no direct knowledge of the view.
- The user interface waits for further user interactions, which begins the cycle anew.
By decoupling models and views, MVC helps to reduce the complexity in architectural design, and to increase flexibility and reuse.
As a design pattern
MVC encompasses more of the architecture of an application than is typical for a design pattern.
Model
The domain-specific representation of the information on which the application operates. Domain logic adds meaning to raw data (e.g., calculating whether today is the user's birthday, or the totals, taxes, and shipping charges for shopping cart items). Many applications use a persistent storage mechanism (such as a database) to store data. MVC does not specifically mention the data access layer because it is understood to be underneath or encapsulated by the Model.
View
Renders the model into a form suitable for interaction, typically a user interface element. Multiple views can exist for a single model for different purposes.
Controller
Processes and responds to events, typically user actions, and may invoke changes on the model.
Excerpt originally from http://en.wikipedia.org/wiki/Model_View_Controller