An Ardour session is stored in a single folder on your computer's filesystem. This makes backup very easy — any tool capable of backing up a folder can be used to backup a session. You pick the location of a session when it is created — by default it will be in your default session location, which can be altered via Edit > Preferences > Misc > Session Management.
There is one complication: a session may reference media files that are stored outside of the session folder, if the user has opted not to select Session > Import > Copy to Session during import. Backing up a session with embedded files will not create a copy of the session containing those files.
The single folder approach also makes sharing a project easy. Simply copy the session folder (onto a storage device, or across a network) and another Ardour user (on any platform) will be able to use it. The limitation regarding embedded files applies to session sharing as well.