Microsoft's Insider Program is the initiative Microsoft used to test and shape Windows 10 through external customer involvement. Anyone can sign up and run the latest beta Windows builds from Microsoft. It is not always a stable way to run your system but is available to anyone and a way you can help shape the future direction of Windows 10.
Should I run Windows 10 Insider Preview?
No. If you ask a random collection of people in an IRC channel, no you should not run it. Read the Getting Started on it and decide if it meets your requirements.
Wow, I am running the Insider Preview and it's unstable!
Did you fail to read the program details? Yes, insider builds will at times be unstable, if you are unprepared for this (backups, recovery process, willingness to contribute feedback through the Feedback Hub app) then please don't run Insider version and complain in IRC channels.
Can I get Windows for free through the Insider Program?
No. You must run a Windows Genuine activated version of Windows.
What are fast or slow release builds?
Twitter posts / updates on the Windows Insider Program
As to why Windows 10?
NT side: Windows NT3.51, NT4, 2000, XP, Vista, 7, 8, 10.
Consumer side: Windows 3.1, 3.11, 95, 98, ME, XP, Vista, 7, 8, 10