HTML is the language of the World Wide Web. If your mail program allows you to change the colour, font, size or any other aspect of the text you are writing, you are probably posting to the list using this document markup standard. Plain text is just that. HTML more than doubles the size of your post, increases the time it takes to download messages in the group, and makes it difficult for people with non-HTML enabled mail programs to read. Consult the documentation provided with your email package to disable this feature.
Flames are heated discussions which are usually personal and almost certainly off-topic.
They can get big very quickly, and that's called a flamewar. If you see a message on the list which you personally object to, please exercise restraint. Remember that this is a forum about Linux in Toronto and that nobody subscribed to read arguments. SPAM is unsolicited bulk communication of any sort. There are many people who have written about Internet etiquette, everything from emotionicons to posting in all caps.