Google Chrome OS is a Linux-based operating system designed by Google to work exclusively with web applications. This means that when you turn it on you don't boot into a traditional operating system like Microsoft Windows or Ubuntu, you boot into Chrome OS.
The best way to explain this is how Google have done so in this video:
Even on a three year old netbook Chrome OS boots up in less than 15 seconds. It's fantastic.
Feel free to leave comments and/or questions, any feedback is appreciated.