There's not much to say about the article window. The article window shows you a particular article. There, that's about it.
Seriously, there's also the fact that from the article window you can respond to that article. There are three main ways to respond to an article: by posting a followup article on the same newsgroup, by replying to the article with an e-mail message to the original author of the article, or by both following up to the newsgroup and sending an e-mail to the author. If you want to post a follow-up message to the newsgroup, click on the "Post Followup" button, or hit [COMMAND]-[F]. If you want to e-mail a reply, click on the "E-mail Reply" button, or hit [COMMAND]-[R]. If you want to do both, click on either button (you'll be able to modify where the message goes in the Post Article window), or hit [COMMAND]-[B].