The .NET Framework is an integral Windows component for building and running the next generation of software applications and Web services.
More info in http://msdn.microsoft.com/netframework/technologyinfo/overview/
Also in http://www.microsoft.com/net/basics/whatis.asp
Do you need it? It depends what you do or what application/program you are using or will use. Some applications requires it. If you are not using software that require .NET Framework, you can uninstall it in Add/Remove Programs
I received this in the Critical Updates yesterday. I downloaded it but am not sure what it is for.... Should I keep it or uninstall? And if I should uninstall, how would I do that?