Is it possible to browse my iTunes backup files? I use to store older backups of my iPad, iPod and iPhone made using iTunes.  

So, as long as some data (i.e: photos) is stored on one of these backups, I feel free to delete it on the devices.  

If, in a future, I need only some specific data (say music, document, video, email... etc), may I access to the backup to...? :


Browse it to see what is there inside.
Extract some specific info.


If yes, what kind of data could be extracted? :  


Only files like photos, videos or documents?
Or is it possible to dig for some more complex data like SMSs, notes, emails, phone lists... etc?


Remember that I don't want to restore any backup. I just want to look inside it.  Also, I don't want recommendations on software that reads the files. I'm looking for how to understand what's on the filesystem.
Yes! It is possible to view the contents of backups. There are several programs you can use, the one I use is iExplorer v3.

You can view and extract:


Messages
Contacts
Voicemail (though I have not tried this)
Call history
Notes
Safari (bookmarks and history)
Calendar
More if you are willing to dig into folders


The free version allows you to view the contents, but has restrictions on extracting them.

There is also iPhone Backup Extractor which does:


Photos
Recordings
Again, perhaps more


This is also a free / paid app, so have a look at what you need.

Hope this helps.