Read: http://learn.elgg.org/en/stable/admin/backup-restore.html.

To be able to restore your site you will need the complete Elgg install folders with all subfolders, the database and the complete data directory. You only need to backup the install folder after changing the Elgg version or upgrading plugins. The database and data directory contain all the user data and you will have to make regular backups of both. The database can only be backuped as a whole, so no way to reduce the work here (you could only try to automize it in some way). The data directory backup can be reduced in the time necessary for example by using rsync to only download the changed/new files from the server, i.e. syncing the local backup with the remote data directory. Other possibilities for backups could be usage of a backup service by your webhoster (if offered).