Tip - What Is the .NET Framework, and Why Do I Need It?

Dear Lifehacker, What is the "Microsoft .NET Framework" and why do I need it? I've been trying to install BlackBerry Desktop, but it fails and states I'm missing the Framework. For some reason I can't download it from Microsoft. What could I do? Got any other advice about .NET Framework and its ins and outs? Drop them in the comments to help out a fellow reader, now and into the future.
Tags: Ask Lifehacker, .NET Framework, Windows, .net, Troubleshooting, Annoyances, Installation, Installer, Top, Lifehacker