Although Alt+Tab by default lets you switch windows by default, while making various tweaks and changes you may unintentionally deactivate it, as it has happened to many new Ununtu users. Since it’s a feature you use it all the time without evening realizing it, if you can’t alt+tab between different windows it can pose a serious problem.
In case you’ve accidentally deactivated this feature, you can easily activate it.
- Press ALT+F2
- type “gconf-editor” (without quotes) and press Enter
- Go to /apps/metacity/global_keybindings
- Find Cycle Windows
- Change the key value to
(you’ll need to press Enter)