Windows 10 technical preview build 9926 is available for developers. You need to join the Windows Insider Program to download the latest version of Windows 10. There are lots of changes compared to Windows 8.1. I have been using Windows 10 for the last 2 months now on my Surface Pro 3.
You see the new action center / notification bar which are new to Windows.
Concept of Windows 10
Starting from Windows 10, Windows would be like a service and the users would be getting upgrades periodically. Windows will have universal Apps which would work on a PC, tablet or a windows powered Smartphone. It would be a free upgrade for existing Windows 7 or 8.1 users. This concept would work great if you use Windows 10 on all your devices which get everything Synchronized. You will have your files synchronized using OneDrive, settings in sync.
Would need to wait and see more, currently the technical preview is not good for real production devices. I see some problems with the Modern UI apps like the Camera on the Surface Pro 3 is not working for me.
Please feel free to share your experience on Windows 10 if you are using it.
More Information on Windows 10 Technical Preview