The name “.NET Framework” itself is a bit of a misnomer. A framework (in programming terms) is really a collection of Application Programming Interfaces (APIs) and a shared library of code that developers can call when developing applications, so that they don’t have to write the code from scratch. In the .NET Framework, that library of shared code is named the Framework Class Library (FCL). The bits of code in the shared library can perform all kinds of different functions. Say, for example, a developer needed their application to be able to ping another IP address on the network. Instead of writing that code themselves, and then writing all the little bits and pieces that have to interpret what the ping results mean, they can use code from the library that performs that function.
And that’s just one tiny example. The .NET Framework contains tens of thousands of pieces of shared code. This shared code makes the lives of developers much easier because they don’t have to reinvent the wheel every time their applications need to perform some common function. Instead, they can focus on the code that’s unique to their applications and the user interface that ties it all together. Using a framework of shared code like this also helps provide some standards between applications. Other developers can make sense of what a program is doing more easily and users of the applications can count on things like Open and Save As dialog boxes working the same in different applications.
Because in addition to serving as a framework of shared code, .NET also provides a runtime environment for applications. A runtime environment provides a virtual machine-like sandbox in which applications run. Many development platforms provide the same kind of thing. Java and Ruby on Rails, for example, both provide their own runtime environments. In the .NET world, the runtime environment is named the Common Language Runtime (CLR). When a user runs an application, the code for that application is actually compiled into machine code at runtime and then executed. CLR also provides some other services, such as managing memory and processor threads, handling program exceptions, and managing security. The runtime environment is really a way of abstracting the application from the actual hardware on which the application runs.