There are three
This tool is selected by default when paint.net starts. It is used to create spots or stripes of color just as a paint filled brush would. For more information on this tool see the Paintbrush Tool section.
As the name suggests, this tool is used to erase areas of the active layer. It erases by setting the opacity, or alpha, values of pixels to zero. For more information on this tool, see the Eraser Tool section.
This tool mimics a hand held pencil, allowing pixel-by-pixel coloration of the active layer. For more information on this tool, see the Pencil Tool section.