Junk is the term Thunderbird uses to describe spam emails. It is chosen to differentiate and to be suitable from translation.

SPAM is the term used by most mail providers and anti virus programs to describe spam. Because of this anything with the word spam in it originates outside of Thunderbird. either as an imap folder or something created by an add-on.

Trash is exactly what it says, other terms that are interchangeable as deleted and "deleted items"

The folders used for junk mail and deleted items in thunderbird are user configurable . Right click the account in the folder pane and select settings. In Server settings you can specify the folder used for deletions. n Junk settings you can specify the folder to be used for Junk/SPAM

According to this reply, spam and junk are one and the same thing. If that is the case, why does Thunderbird have BOTH spam and junk folders? I continually get wanted emails diverted to spam (not junk). How can I reset the spam filter to accept wanted emails into the inbox?

I guess you missed the part where Thunderbird uses the work Junk to differentiate itself from items that an outside source marks as Spam. Thunderbird does not create or use a Spam folder or mark messages as Spam.

Many thanks for a quick reply. OK, so my ISP classifies it as Spam and Thunderbird classifies other stuff as Junk. I can ask Thunderbird to reclassify mail from a particular source as Not Junk, but how can I get mail in the Spam folder reclassified as Not Spam?

Figure out who or what is adding that tag and make adjustments. It is not Thunderbird.

How do I do that?

If it is your ISP that is classifying it as spam ask them for advice on how to manage their spam tools. If it is your "internet security suite" I suggest you contact that supplier.