This is where you can select most program options. Detailed information here.
Toolbar:
This is where you can select the actions to perform on a group or post. Detailed information here.
Thread status:
This is where you can see what each thread (threads are used for multiple connections, one thread for each connection) is doing. Detailed information here.
Group list:
This is where the list of your currently selected groups appear. Detailed information here.
Header list:
This is where the list of currently downloaded headers is shown. Detailed information here.
Status bar:
This is where the current status of the program is shown, and where you can select your view: headers, queue or log. Detailed information here.