Al Idraj is a saying not of the Prophet's (May God's blessings and peace be upon Him), and is sometimes on the part of the narrator, either to clarify a word or to make a saying of the Prophet (May God's blessings and peace be upon Him) more complete. A Wad' means fabrication. So, what is the difference between Al Idraj wal Wad' (الإدراج و الوضع)?
The difference is simple:
In case of Al Idraj, the narrator does not claim that the part the explains or clarifies the hadith from the prophet words -peace be upon him-. In many hadiths the distinguish is made explicit, for example after narrating the hadith the person says and
However, in the fabricated hadith the narrator completely make the entire hadith from his/her mind and claim it to be from the prophet sayings -peace be upon him- .