The Paint.NET user interface is broken up into 10 major areas:
This tells you the name of the image you are working on, as well as the current zoom level and the version of Paint.NET you are using.
This is where you can access the various menu items. Quite often, commands accessible from this location will be referenced using Menu → Command notation. For example, File → Exit means to click on the File menu, and then click on the Exit command.
Immediately below the menus is the toolbar. It contains several buttons and controls for executing various commands and adjusting parameters for the active tool.
Each image that has been opened has a thumbnail displayed in the image list. This works much like a tabbed web browser: just click on an image to switch to it.
This is where the image is shown, and is the area where you may draw and perform other actions.
This where the active tool is highlighted, and where you may choose from other tools.
Everything you've done to an image since you opened it is listed in this window. You may step forward or backward in the History window by clicking an entry.
Every image contains at least one layer, and this window is your primary area for managing them.
This is the primary area for selecting colors to draw with. It consists of a color wheel and a brightness slider. If you have expanded the window with the "More" button then it will also contain several controls for fine tuning and exactly specifying color values.
This area is divided into several sections. On the left, it displays quick help and status information. On the right, it displays rendering progress (if pertinent), image size, and the cursor location within the image.