Quick backup strategy to an external server

Hello, before ending my trail period and buying your fabulous suite I still wonder how backup works.
I have read some pages on wiki guide but still not understand how it works.

I have initialisez and perform an scan.
It has created a new folder backup/zextras with lots of data. Are this data the realtime backup ?

Then what is default and defaultexternal ? what is the difference ?

What I want to do is to copy to an external storage via rsync over ssh for instance
I can create a schedule task.
But what to do ?
Should I create first a task for export then to copy via rsync ?

Sorry to ask so much questions. I thought Zextras backup was easyer and I'm not very familiar with Zimbra admin.