This window is split into two sections - on the left is a list displaying the newsgroups you have opened, and on the right are the messages inside the currently selected folder (either a group, 'sent' or 'outgoing'. The columns in each list may be resized, and their order rearranged.
Groups list:
This has three columns. Name (either the Description entry, or the actual newsgroup name if you left Description empty); Messages is the number of messages in that folder; the third column shows an 'S' if you are currently subscribed, and is blank if not. You can change the selected group by clicking on an entry (the messages list will update automatically).
Doubleclicking on an entry opens the Edit Newsgroup window for that group (you cannot edit 'sent' or 'outgoing'; you can only doubleclick on newsgroups).
Messages list:
This has four columns. The first one is a status column - it displays 'N' in newsgroups if the message has not been read yet, messages in 'outgoing' display 'H' if they are Held (ie, will not be sent upon selecting 'Send News') - a '*' indicateds that the message was downloaded in online mode, and the body has not yet been downloaded; the second column is the Subject of the message; the third is the Date; the fourth is the From field of the message if you are currently viewing a newsgroup, or if in 'outgoing' or 'sent', it shows the Newsgroup(s) that the message will be posted to.
You can choose how messages are sorted by clicking on columns (not the first, 'Status' column, though); the messages will then be sorted by that column. By default, messages are sorted by date.