ASP.NET is basically "web programming" using visual basic in the background to do all of your web logic and data handling to build custom web pages based on the user's input, and the data in your own local database.
ASP.NET is the Microsoft answer to Apache/Linux's PHP. PHP is a scripting language that is modeled on C/Java syntax, and was originally only able to be used on Linux/Unix servers. (There are now Windows versions of the PHP server for IIS).
The way PHP and ASP work is pretty different... and one big benefit of ASP is the use of the visual studio editor, whereas PHP is largely done in a regular text editor without any visual styles, etc... Also, ASP is more like a program, and PHP is more of a script.