Tool Window: What's This?

Paintbrush: Draw with whatever is displayed in the brush view
Fill: Click on a pixel and all neighboring pixels like it will also be changed to the new color
Line: Paints a line using the current brush
Spline: This does not work yet. When it does, you will be able to easily draw curved lines

Rectangle: Draw a rectangle or square using the current brush
Ellipse: Draw an ellipse or circle with the current brush
Text: Brings up the text window
Zoom: One of the several ways of zooming in or out on the image

Eyedropper: Click on a pixel in the image with either mouse button and the current foreground or background color will become that color
Stamp: Copy part of the image to another part of the image. The right mouse button will choose the part of the image to copy from. The left mouse button paints. Otherwise, it operates just like the Paintbrush tool
Magic Wand: Operates just like the Fill tool except that colors are selected. Doesn't work yet.
Eraser: Places you in erase mode. When in erase mode, the regular tools erase instead of draw. Filters will also erase.

Filled Rectangle: Like Rectangle, but the rectangle is filled in
Filled Ellipse: Like Ellipse, but the ellipse is filled in
Mask: Places you in Mask mode, allowing you to edit the selection with any of the tools or filters
Move: Doesn't do anything yet.