Ubuntu one is a free on line file sharing service offered by Canonical. The newly released ubuntu 9.10 has Ubuntu One tightly integrated with it. For using this service , you have to sign up with Ubuntu one. Click on this link and fill in the details to get a free account which can hold upto 2 GB. A mail will come to you mail account asking you to confirm the subscription.
Once you have completed the registration and confirmed , the ubuntu one account can be set up on your karmic koala desktop.
Click on Applications >> Internet and click Ubuntu One. The following window will pop up.
You can set the preferences for show icon ,connect on start and bandwidth.
The web browser will be launched and you will be taken the the launchpad website and asked to login. Once you login you will again be asked to confirm whether you want to add you computer to Ubuntu One. Once you agree, the Ubuntu one menu item will come under . See the picture below.
You can use this folder as any other normal folder on your computer.
This folder will be under your home directory. If you open a terminal and move to ~/ Ubuntu\ One/ folder, you can see the off-line contents on your hard disk. If you open the folder in nautilus, a special connect / disconnect button appears. You can use this to connect to the Internet and update your files on the Ubuntu one servers.