Some of the participants were a little bit fuzzy about the differences, so I thought I would put down a few thoughts on the subject since I collect a lot of this junk.
Junk is good because junk is cheap.
Precancels are stamps cancelled prior to use The operative words are prior and use. They are also linked. The cancels are either printed or rubber stamped and their application is done before use. The purpose of precancels was to speed the mail by avoiding the canceling process at the post offices where they were mailed. They were generally used for the mailing of newspapers except in the US and Canada where they were used for first class, parcel post and bulk mail.