When you first install Carbonite you are given two options for backup:
Automatically Back Up My "Users" Folder and Desktop - This option selects the data files located in your Users folder, which includes the files and folders in your Desktop folder.
I'll Manually Select What to Back Up Later - This option does not select any files to be backed up. Instead it lets you choose what to back up after the installation.
By default, Carbonite does not back up system files, executable files, or temporary files and folders. This is to avoid problems in the event you need to restore your files to a new computer. If you restore outdated system files, they could overwrite the new files that are managing the operation of your computer.
Carbonite also does not back up video files by default. However, you can easily back up most files you want through the Carbonite Preference pane.
Technical Details
Your Carbonite subscription is associated with a backup policy which determines whether a file is, by default, included or excluded from your backup. Carbonite backs up all files except for:
Application files (an exhaustive list of these extensions is available upon request).
Files in known system and temporary folders. (These files cannot be added to your backup for your protection).
Files contained in hidden system folders (a.k.a super-hidden folders).
Files which are over 4GB in size (however, you can individually add large files to your backup through the Carbonite Preference pane).
Video files (but you can add video files to your backup within the Carbonite Preference pane.)
Files on non-permanent disk drives (e.g., network and removable USB drives) unless otherwise noted as a feature in your Carbonite product.
Carbonite also offers products that have limited storage. These products, due to either restrictions on files that can be backed up or due to storage limitations, may not be able to back up all of your important files. For more information about what files or folders are backed up with these limited products consult the specific product offer.