This information was provided by Tobias Koenig, KDE-PIM developer.
Een paar definities
By 'real data' we mean the data, like the contacts or events. These data are stored either on a groupware server or in local files. Where exactly depends on the resource you are using. E.g. the 'Personal Contacts' resource stores its data under $HOME/.local/share/contacts while the 'VCard File' resource (which is not recommended to use anyway ;)) stores its data under $HOME/.kde/share/apps/kabc/std.vcf.
The 'cached data' are copies of the 'real data' that are kept in the database for faster access and offline caching. The database also keeps the 'meta data' which are management data needed by Akonadi to work correctly.
The 'configuration data' are the data that configure the Akonadi server and the individual resources. The configuration data for the server can be found under $HOME/.config/akonadi, the configuration for the resources are mostly stored under $HOME/.kde/share/config/akonadi_xyz_resourcerc# (xyz is name of resource and # its instance number).
So now we need to decide what to back up. If you want to backup the 'real data', then it depends on the resources you have configured... if you use a groupware server, then the backup should be done there. For contacts, the files under $HOME/.local/share/contacts will normally be what you need.
The Akonadi tray icon offers you a backup and restore function. That backs up the 'cache data' which might not include all of your data! It can be helpfull if you want to clone a system 1:1, then you can copy the cached data together with the real data and the configuration, so you don't have to load the data again from the sources (e.g. groupware server).
Examining your Resources
KRunner offers you Akonadi Resource Configuration, or you can access this through the Akonadi tray icon > Configure. You may find several resources set up. You may find one labelled
Address Book - No KDE address book plugin configured yet.
That's the old compatibility bridge (possibly created by the migrator tool). You should remove this one!
std.vcf - Ready
This is the 'VCard File Resource' which points to $HOME/.kde/share/apps/kabc/std.vcf per default. It is not recommended that you use that one, as it doesn't share the benefit of Akonadi.
Personal Contacts - Offline
That's the preferred resource for your local contacts which points to
Note that this may say 'Offline' when in fact you are using it. This is a display bug, and can safely be ignored.
And when more components use Akonadi?
When the rest of the components move to Akonadi, will all the data be under ~/.local/share? That should make backup easy. Well, contacts and mails are stored there now, I don't know what the plan for events/todos is, but it would make sense to store them there as well. We'll keep it in mind when discussion pops up!