MVC is a design pattern. A reusable "recipe" for constructing your application. Generally, you don't want your user interface code and data access code to be mixed together, it makes changing either one more difficult. By placing data access code into a "Model" object and user interface code into a "View" object, you can use a "Controller" object to act as a go-between, sending messages/calling methods on the view object when the data changes and vice versa.

Model-view-controller (MVC) is an architectural pattern used in software engineering. In complex computer applications that present a large amount of data to the user, a developer often wishes to separate data (model) and user interface (view) concerns, so that changes to the user interface will not affect data handling, and that the data can be reorganized without changing the user interface. The model-view-controller solves this problem by decoupling data access and business logic from data presentation and user interaction, by introducing an intermediate component: the controller.

Model:
    The domain-specific representation of the information that 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.