One additional difference between GMail/AppleMail "conversations" and Thunderbird/Evolution "threading" is that the formers do merge conversations from different IMAP folders (as long as the auto-threading rules kick in), and do merge duplicate mails automagically (e.g. if you have the same mail in "Sent" and in "Mailing List Foo", they will not be displayed twice in the conversation). This for instance allows to have full conversation for emails in Inbox, which is usually not working in Thunderbird since the emails are usually split between Inbox and Sent.
IOW, a "conversation" is displayed in any folder that contains at least one message which is part of it, and contains all messages that are logically part of it, even if they're not stored in that folder. A "thread" is a visual arrangement of the emails in the specific folder, but doesn't cross the folder limits.
Another important point of the user interface is the "mute" button, that hides (archives) the conversation and doesn't allow it to popup anymore even if new messages arrive (in any folder).
IOW, a "conversation" is displayed in any folder that contains at least one message which is part of it, and contains all messages that are logically part of it, even if they're not stored in that folder. A "thread" is a visual arrangement of the emails in the specific folder, but doesn't cross the folder limits.
Another important point of the user interface is the "mute" button, that hides (archives) the conversation and doesn't allow it to popup anymore even if new messages arrive (in any folder).