I was trying to find the meaning of those things but the only thing I found was "how to add data to groups". Forgive me If I am wrong but I am trying to understand what's the difference between folders and groups and cannot see any documentation on this. Why can I add a group in a folder and not the opposite ? Regards
If you are referring to the help files, there is a topic on this, search for "about groups and folders" (or Browse Together Help > Working with Groups and Folders > About Groups and Folders) that explains this in detail.
In a nutshell, groups are loose collections of items, an item can be in more than one group at a time (like an iTunes playlist or iPhoto album) and folders are just like folders in the Finder, so an item can only be in one folder at a time and can have subfolders.
As it was a very popular request, groups can be nested in folders, but not the other way around. Placing a group in a folder does not affect where the files in the group are stored.