RAW item :
A raw item is an original file.
EDITED item :
An edit session is a virtual file.
Several types of edited item exist (depending on the tool you use for editing).
: edit session coming from the "Snippet" tab.
: edit session coming from the "Multitrack" or the "Snippet+" tab.
: edit session (called "edited item") coming from the "Production" tab.
Attention : Do not transform an "edited" item into a Master, otherwise you will loose the information that you have put on the sound 
.
MASTER item :
A master is the transformation of a edited file into a real sound.