The ASP.NET MVC Framework is a Model-view-controller framework which Microsoft is adding to ASP.NET. It allows software developers to build a Web application as a composition of three roles: Model, View and Controller. A Model represents the state of a particular aspect of the application. Frequently, a model maps to a database table with the entries in the table representing the state of the table. A Controller handles interactions and updates the model to reflect a change in state of the application. A View extracts necessary information from a model and renders a user interface to display that.[1]
The ASP.NET MVC Framework couples the models, views and controllers using interface-based contracts, thereby allowing each component to be easily tested independently. By default, the view engine in the MVC framework uses regular .aspx pages to design the layout of the user interface pages onto which the data is composed. However, different View Engines can be used (for example, you can create a View Engine based on XSLT files[2]) Additionally, rather than the default ASP.NET postback model, any interactions are routed to the controllers using the ASP.NET 3.5 SP1 Routing mechanism. Views can be mapped to REST-friendly URLs.[1]

http://en.wikipedia.org/wiki/ASP.NET_MVC_Framework
http://www.asp.net/mvc/