As the name ASP.NET MVC already suggests there are at least three components defined; namely the Model, View and the Controller. But lets first see what the main responsibilities of a regular data driven application are:

• Data Representation: the model or actual data needs to be represented in a form the user-agent is able to consume. This information is sent in the HTPP request header by defining an 'accept' field. This field contains a semicolon-separated list of representation schemes and if missing the default value is 'Accept: text/plain; text/html'.
• Look&Feel / Appearance: no-one likes to look at raw XML or HTML and so we send back stylesheets in our response together with our Data Representation. The user-agent (for example a web browser) then transforms the raw data into a look&feel that our ui-designers came up with. 
• User Experience: many successful web sites (Google Search) are complete with the HTML and CSS defined above and in fact every web site should be usable without any Javascript & Ajax technology applied. Think of JavaScript and Ajax as browser side technologies to add bells & whistle for an improved user experience.
• Business Logic: This term is probably ranked in the top-ten of most misused and misunderstood terms in computer science. In my definition of Business Logic, I exclude any business requirements for the user interface. Business Logic is a backend term and has two categories: filters and validators. Business Logic filters are applied whenever a model is requested from the backend and most of the time contain rules about the requesting user. I.e. only Model instances a user is allowed to see are returned. The other category of Business Rules are the validators and are enforced before Models are persisted back to the data source. For example, a Model has to be in a certain state in order to accept values or Model attribute values have to conform to specified rules.
• Persistence: Models are objects in memory and since object-oriented databases didn't make to mainstream (yet), one layer of an application is concerned with mapping objects state to relational tables in a database and vice versa.