As others have said it is the phone notifying you about something. The Droid sound is the default for all notifications, I think (don't quote me on that). I know that it was so on my droid. Most programs that include notifications have the ability to have a sound attached to them, either from the system library or from an outside source either imported or created by you. Personally I went through most of my programs that have notifications and assigned each program (and in some cases each account), different sounds. It took a while to remember which was which, but now when a specific sound rings, I know which program and/or account it is and whether i want to check it right away or not.