In Thunderbird you can tag a message with one or more tags by clicking on the message with the right mouse button and choosing Tag. There are five standard tags with descriptions Important, Work, etc. If the mail server is an imap server like post.science.ru.nl, then the tags are stored on the server, which means that every Thunderbird will show the tags.
It is somewaht more complicated for new tags that you can create. Thse are also stored on the imap server, but will initialy not be visible in other Thunderbird programs. One can make a new tag by clicking with the right mouse button on a message and choosing Tag -> New Tag... or using the menu Tools -> Options... -> Display -> Tags -> Add. The tag name will be used both as internal tag name and as visible description.