choosing programming model in asp.net

There are 3 flavors of ASP.NET

The first one is the oldest and is called Web Forms. Basically it is a high level component oriented web framework that works with controls like buttons and grids that encapsulate behaviour and view. It was the most popular flavor of ASP.NET but has been criticised for the lack of control over the generated markup. Currently most new projects are ASP.NET MVC but there is definitely a lot of Web Forms out there. While this is my personal favorite I must point out that it is a bad way to start learning web programming because it hides the implementation details from you (which is good when you have experience) and is a bit complex to learn.

Source : http://www.asp.net/web-forms

ASP.NET MVC is an implementation of the MVC pattern for ASP.NET. Some people claim that it is easier to develop maintainable applications with unit tests and good separation of concerns with this framework than it is with Web Forms. I disagree on this point and think that using patterns like MVP one can achieve the same with Web Forms. On the other hand ASP.NET MVC has one big advantage - it allows full control over the generated markup. This is very important for the modern style of web development where a lot of things are controlled with JavaScript. For example adding a fancy animation is easier to do on top of an MVC view than it is on top of a Web Form.

Source: http://www.asp.net/mvc

ASP.NET Web Pages is a (currently) the latest flavor that is targeted at smaller project and beginner devs (at least in my opinion). It is good for developing smaller projects with ~ 10 pages. Most of the logic is written in a single file per page in what I call "Basic PHP style". It uses the Razor syntax for injecting the serverside code.

Source: http://www.asp.net/web-pages

Note that Web Forms uses pages (unlike MVC) therefore there is a confusion what ASP.NET Web Pages is