Google Chrome allows extensions and hosted apps on Chrome web store to use background pages to display timely notifications for e.g Google Mail Checker extension uses background pages to display number of unread messages in your Gmail inbox. These background pages are invisible as they constantly runs in the background.
Some apps keep running even after you close Chrome, these are called background apps for e.g recently covered Google Music Player Sample. If background apps installed for Chrome, you can view a Chrome icon in the system tray which lists background apps running when you right-click on it.
Managing Background apps and pages
These background pages and apps can also consume system resources, so Google Chrome allows to users to manage them. If any extension or app using background pages Wrench menu displays view background pages item, click on it opens task manager highlighting apps and extensions with background pages running.
The Task Manager displays memory and CPU usage for these apps, you can select any background app and click on End process button or you can select Exit from wrench menu to close Google Chrome completely when background apps are running.
You can disable or uninstall extensions also. To disable all background apps from running after closing chrome you need to open Options>Under the Hood uncheck for continue running background apps when Google Chrome is closed.