The main window shows a list of the packages available. You can filter the list by selecting a category from the drop down on the toolbar or clicking on Find and searching for text in the package names and descriptions.
The left hand column is either:
The ticks have a small "a" by them if the package was automatically installed.
This is followed by the package name and a summary of what it does.
Clicking on a package will show more information about it in the status bar at the bottom of the Window. The small triangle on the right of the status bar allows it to be shrunk (and expanded afterwards) to allow more packages to be shown in the main view.
To install or remove a package after selecting it click on the Install or Remove button on the toolbar (the first two buttons respectively) or the menu.
You will be prompted with the Install Window or Remove Window to confirm you are sure you wish to install or remove the package before the installation/removal proceeds.
Once a package has been installed or removed the main window will be updated to show it's new status
Using the Menu button on the main window shows a menu with the following options