How do I find and backup Thunderbird e-mail files and folders

I want to find and backup my Thunderbird e-mail files as well as the Thunderbird folders I have placed them in but don't know the file extension for Thunderbird e-mails or where they are located. Also I don't know whether the Thunderbird folders are true folders on the XP ioperating system and if so how do I find them. If not how do I keep my various e-mails in their respective categories that I created with my Thunderbird folders.