The main window contains everything you need to use proNotes. proNotes has the ability to work with multiple windows (check out the "Note Window" section of the full documentation) but proNotes was designed as a single-window application.
The major parts are the category table, title table, details table (the three boxes on the left) and the preview pane (the section on the right).
The buttons on the bottom of the window let you change what you see in the different sections of the window.
The snapshot bar holds snapshots which let you save a particular window configuration (i.e. a selected category, title, detail and preview pane) just like a bookmark would in your favorite browser.
The search field will narrow down what you see in the main window based on what you type.