What are the .NET Frameworks?
The .Net Frameworks are a one time install that allow programs written in Visual Studio 2002 and newer to run on computers without needing to distribute large files.
The .Net Framework is best described as an abstraction layer between the operating system of a computer, and a piece of software which was built to run on said framework. The abstraction layer provides overall better system stability, because software is running against this framework, and not directly against the operating system API. Certain things that generally lead to instability, like poor memory management, is taken over and dealt with by the framework.
If you have Windows XP then odds are you already have them all installed courtesy of Windows Updates. If you have Windows Vista or Windows 7 then you already have all the Frameworks up to v3.0 installed since the OS itself uses .Net
For a detailed overview of what .Net encompasses visit the Microsoft .NET Framework Conceptual Overview