I am curious about how Ubuntu One automatic file sync works under the hood. When I add or change a file in my file manager, how does it know that the change occurred and it needs to sync it? This will likely need a somewhat technical answer.
The Linux kernel supports something called Inotify. Using it, you can tell the kernel to notify you when something happens to a file or folder. So you can do something like "Whenever a new file is placed in this folder, or whenever a file in this folder has been changed, run the following action".
You can be notified when a file is created, read, written to, opened, closed, moved (or renamed), deleted, etc.
I don't know for a fact that this is how Ubuntu One file sync works, but I think it's a good assumption. If you're interested in the techniques, I suggest reading up on Inotify.