If you're familiar with Usenet, the Newsgroup window should be easy enough to use. If you're a first-time traveller through Usenet territory, on the other hand, it might be a bit confusing.
First, an explanation. Every article (a.k.a. message) on Usenet is made up of two parts: a list of headers, and the actual body of the article. The headers contain basic information about the article, such as the subject of the article ("Taking care of sheep"), who wrote the article ("gms@ucom.com"), when the article was written ("October 3, 1999"), etc. The body of the article contains the actual text of the article ("I think that....").
However, Usenet contains a vast assortment of discussions, some useful, and some that are rather less than useful. As a result, you probably won't want to read all the articles on a particular newsgroup (even if you could; some newsgroups get hundreds of new articles every day). Thusly, there's no reason to download every single article in the newsgroup. Fortunately, you don't have to.
The Newsgroup window is divided into two parts. The upper half is basically your table-of-contents for a particular newsgroup. It lists all of the articles in the newsgroup, including who wrote them, when they were written, etc. In order to do this, the Newsgroup window doesn't bother with the actual bodies of the articles; it just manages the headers of each article. The lower half displays an actual article, provided you've downloaded it.
OK, so when you click on the "Refresh" button to download new messages from your news server, Informant will only download new headers and display the information in those headers. No point in downloading the entire article, when only the headers will do.
The Newsgroup window is geared towards letting you manage a particular newsgroup. If you want to reply to a particular article, you can reply to it from here. If you want to post a new article to this newsgroup (without necessarily replying to any other article), you can do that from here, too.
If you want to see the body of a particular article, select it in the list of headers, and click on the "Get Body" button. Informant will start downloading the article, and when it's done, it will display that article in the lower half of the window.
To view a particular article in detail, double-click that article's listing in the list of headers.