The Tools Window is where you choose the active tool, which may then be used to edit the image.
Rectangle SelectYou may use this to define a rectangle or square selection region. For greater discussion of how to use this tool, please visit the Shape Selection Tools section.
Lasso SelectYou may use this to draw a freeform selection region. For greater discussion of how to use this tool, please visit the Shape Selection Tools section.
Ellipse SelectYou may use this to draw an ellipse or circle selection region. For greater discussion of how to use this tool, please visit the Shape Selection Tools section.
Magic WandYou may use this to select areas of the active layer that are similar in color. For greater discussion of how to use this tool, please visit the Magic Wand section.
You may use this to move pixels that are currently selected as a result of using the various Selection Tools.
Move SelectionYou may use this to move the selection without affecting the pixels that are selected.
ZoomThis tool can be used to zoom in (left click), zoom out (right click), or zoom the whole canvas around a particular region (draw a rectangle).
PanUsing this tool you can scroll, or pan through the image. This functionality is also usable from any other tool at any time: simply hold down the spacebar and then click and drag with the mouse.
Paint BucketThis tool is useful for filling in areas of similar color with a different color. For greater discussion of how to use this tool, please visit the Paint Bucket Tool section.
GradientThe Gradient Tool allows you to draw gradients (a gradual blend of one color to another) in several formats. With its transparency mode, you can also "fade" or "blend" two images together. For greater discussion of how to use this tool, please visit the Gradient Tool section.
PaintbrushThis tool is selected by default when you start Paint.NET, and is useful for many kinds of freeform drawing. For greater discussion of how to use this tool, please visit the Paintbrush Tool section.
EraserYou may use this tool to erase areas of the image (it sets the opacity, or alpha, to 0). For greater discussion of how to use this tool, please visit the Eraser Tool section.
PencilThis allows you to edit the active layer pixel-by-pixel. For greater discussion of how to use this tool, please visit the Pencil Tool section.
Color PickerYou can use this tool to pick up a color from the active layer and set it as the current primary or secondary color. For greater discussion of how to use this tool, please visit the Color Picker Tool section.
Clone StampThis tool is useful for copying regions of pixels between layers, or within the same layer. For greater discussion of how to use this tool, please visit the Clone Stamp Tool section.
Recolor ToolThis tool is useful for replacing one color with another. For greater discussion of how to use this tool, please visit the Recolor Tool section.
Text ToolThis tool can be used for placing text on the image. For greater discussion of how to use this tool, please visit the Text Tool section.
Line/Curve ToolYou may draw straight lines and curved lines with this tool. For greater discussion of how to use this tool, please visit the Line/Curve Tool section.
RectangleThis can be used to draw rectangles and squares. For greater discussion of how to use this tool, please visit the Geometric Shape Tools section.
Rounded RectangleThis can be used to draw rounded rectangles and rounded squares. For greater discussion of how to use this tool, please visit the Geometric Shape Tools section.
EllipseThis can be used to draw ellipses and circles. For greater discussion of how to use this tool, please visit the Geometric Shape Tools section.
Freeform ShapeThis can be used to draw a shape with a freeform outline. For greater discussion of how to use this tool, please visit the Geometric Shape Tools section.