Source: http://lifehacker.com/#!5769168/chrome-extensions-now-allowed-to-run-in-the-background
There aren't a ton of extensions using this feature yet, but it's fairly new. We expect that just like desktop extensions, a lot of developers will be adding this feature to their extensions very soon. Essentially, it allows you to start a process and close all your Chrome windows. The process (checking your Gmail, for instance) will stay open in your system tray (or the Dock, in OS X's case), so it can keep giving you desktop notifications even when you don't have any Chrome windows open. You can also access your background-enabled apps from the system tray or dock, which is pretty handy.
This feature actually has quite a few neat possibilities. For example, it also allows Chrome apps to load content in the background too, so apps can update in real time even when they're not open. That means in theory, something like Mint—which is right now, just a bookmark to Mint.com in the Chrome Web Store—could load all that information in the background, ready for you when you open it up, no waiting required.
We're pretty interested to see what developers do with this feature now that it's out. To test it out, you can check out the countdown timer app, which will send you notifications even when you've closed all your Chrome windows. You'll need Chrome 10 for the background feature to work. Hit the link for more information.
You can contact Whitson Gordon, the author of this post, at whitson@lifehacker.com. You can also follow him on Twitter and Facebook.