FreeNAS has a multitude of options for network backups (i.e. backing up your own PC on to the FreeNAS hard drive).
The simplest is to enable file sharing and do backups manually by copying files from each individual computer.
The are both synchronisation software, which means that once enabled correctly, at each subsequent command to backup, they will create an incremental backup (i.e. changes since the last backup). They also automatically delete unwanted files if you delete them on your PC (be aware of this).
So which is best?
Unison synchronises in both directions. i.e. if you change the version of any file on either your PC or the FreeNAS server, it will calculate which is the newer version and keep that copy on both machines. Obviously, this is useful if you edit files directly on the server, which can be necessary if you need to edit the files on multiple different networked PCs.
Rsync only synchronises in the direction you tell it to. i.e. if you sychronise from your PC to the FreeNAS server, all copies of files from your PC will be copied to the server, irrespective of which has the newer version. This is simpler if you never edit files on the server directly. Another benefit of RSync is the very easy installation procedure for rsync software on Windows (and Linux) PCs in your network.