ToolsTools generally use the current foreground colour on a left click and the background colour on a right click. The primitive tools can either be filled or outline. You can select which behaviour using the icon.
Brush ToolThis tool draws with the current brush or if no brush is current it draws a freehand line. It's properties allow you to set the operator used to combine the brush with the document. The set operator overwrites the destination pixels. The alpha operator combines the brush pixels with the document pixels in the ratio you specify. 90% would mean mostly the brush and a little of the document. The linear gradient draws a linear gradient on the background. Set the angle by clicking on the arrow and moving the mouse. The gradient will go from the foreground colour to the background colour. Then select a primitive tool like the rectangle and start drawing. The radial gradient draws a circular gradient.
Select/Paste ToolThis tool allows you to select a rectangular region from which a new brush is created. Once a brush is created the Brush tool is automatically selected to allow you to paste the brush on to the document. If the "Paste Transparent" option is on when selecting an area as a new brush any pixel that is the same as the current background colour is made transparent in the brush. This allows for non-rectangular brushes to be created. If you right click and drag to select an area then the object selected into the brush is removed from the document and replaced with the background colour.
Eye Dropper ToolThe eyedropper will pick up the colour when you click on the document and place it in the current foreground colour (left click) or current background colour (right click).
Magnify ToolWill center the zoomed view around the location clicked in the normal view.
Text ToolThis tools let you create a new brush that contains a peice of rendered text, in the selected font, size and styles. Which can then be pasted onto the document. The letters will be in the current foreground colour.
Flood Fill ToolFills with the current colour till the boundry of the colour clicked on. You can select the sensitivity of the fill so that it fills similar regions of colour or just the exact colour.
Freehand ToolDraws a freehand line in the current colour.
Line/Circle/Ellipse/Square/Rectangle ToolsDraws a primitive in the current colour and fill setting.
Polygon ToolDraws a polygon in the current colour and fill setting. Place each point in the polygon using the left mouse button. Use the right mouse button to complete the object. You can select the polyline segment type in the tool options. As you add points the object will draw little boxes on screen marking the start and end points of each segment, which you can move around with the mouse. If you draw a curve, the control points are in red, and are movable too. When the object is done, click "Finish" to apply to the document, otherwise right click to cancel. Right clicking a point deletes that segment.
EraserThe eraser tool clears the document to transparent if possible. Otherwise it uses the background colour.