what does the "disable" button do? When I go to Settings &gt; Applications &gt; Manage Applications &gt; Downloaded I see an application list. If I select some applications I see button "disable". What does this button mean?  What happens if I press it?
The "disable" buttons will essentially "freeze" a system application.

System applications are the applications that come pre-installed on your phone - unremovable apps such as the browser, the gallery, the people app, and so on. Disabling one of these apps will stop the app from doing anything. It'll be removed from the app menu, other apps will no longer be able to open it, it will stop running in the background if it does, etc. It's like uninstalling a normal app, but you have the option to re-enable the app and bring it back at any point in time.