.net is a core set of programming tools from Microsoft to replace COM, COM+, Visual Basic, and C++ with a better way to get the same programs written.
The centerpiece is an architecture where C#, VB.net and dozens of other languages not only compile to code that runs at the same speed, but also share data types and 10s of thousands of objects (standard objects for debugging, Regular expressions, security and cryptography, graphics display and printing, and much more).
.net offers frameworks to write:
- Windows applications
- Windows services
- Web Applications running on traditional browsers
- Web applications running on mobile devices (palms, cell phones, pocket PCs, etc.)
- Console applications (DOS apps)
- XAML / WinFX Apps
and most importantly the libraries, executables and components are so compatible and interchangeable cross-language object oriented programming is possible without source code. And the resulting compiled objects are portable at their heart -- running on a number of diverse chips and OS.
And to protect your investment it can invoke and communicate with your old COM and COM+ code so you can use past work as part of your .net architecture if needed.