I was just asked by a friend to give an overview of the tools available in GIMP, so I decided to share that with all of you readers.
I will start off with showing the basic groupings of tools, which is shown in illustration 1. The red (first) group is the selection tools, which may be combined into a single tool in the future. Each of these tools is particularly useful in certain situations. The next tool isn't highlighted at all because it doesn't fit into any of the categories. It is the path tool, used to create vector shapes, sometimes useful for marking out shapes, or storing selections. The magenta (second) group is what I like to call support tools, used for various tasks throughout some design projects. The green (third) group consists of transform tools, used to move rotate, align, and skew layers within the image. Technically this group should contain the two tools highlighted in yellow, but I find that those two are usually best used in special situations. In the middle of that group there is a knife icon, which is the crop tool, and directly following it is the text tool. The cyan (fourth) group is the fill tools, the first for solid color, the second for gradients. The blue (last) group is the paint tools, used for repairing the image, and painting.
Within the selection tools, in order: box, circle, free, fuzzy, color, intelligent scissors, and foreground. The box and circle tools are used for basic shape selection and can be useful for image cropping. The free select tool is also shape based, but the shape is user defined, so it becomes really useful for selecting objects or regions within the image that contain varied color or texture. Fuzzy and color select are quite similar, both working off of similar color, differentiated by the fact that fuzzy will only select connected areas. Intelligent scissors follow the edges of brightness or color, and can make selection of complex objects that are differentiated from their background. The foreground select tool is used when there is a single color background, and uses the background color of the pallet to determine what not to select.
I won't elaborate on the path tool for the present.
The 'support' tools are, in order, eye dropper, zoom, and measure. The eyedropper is used to pick colors from the image for use during editing. A quick tip relating to colors is to use the X key to swap the foreground and background colors in the pallet. The zoom tool is becoming less useful, but can serve as a reminder of the Ctrl+scroll shortcut to zoom in/out on the picture. If you have a mouse which allows clicking with the wheel you can scroll around the canvas that way. The measure tool (Figure 2) is pretty self explanatory, it is just used to measure distances and angles.
Transform tools (including special ones) in order are, move, align, rotate, scale, sheer, perspective, flip, and cage transform. I will skip further explanation of move, rotate, scale, and flip, as they are quite intuitive. Align is used to align layers to different layers or guides. Sheer is rarely useful, but shifts the lines within the image one way or another on either the horizontal or vertical axes. Perspective (Figure 3) is sometimes quite useful, such as when correcting lens distortion, or you simply want a somewhat 3D effect for your image. Cage transform allows you to select an area defined by a series of points, and then stretch/modify the area based on movement of those points.
The text tool is used for adding text to an image, useful for posters etc.
The fill and gradient tools are fairly standard also, so I will leave further explanation until I am ready to post an advanced tutorial on their use.
The last grouping of tools, in order: pencil, paintbrush, eraser, airbrush, calligraphy (only useful with a graphics tablet), clone, heal, perspective clone, blur/sharpen, smear, and dodge/burn. The pencil, paintbrush and airbrush tools are quites similar, differentiated by the fact that the pencil doesn't support partial transparency for brushes, and the airbrush doesn't lay all the color down at the same time (how fast is governed by a 'rate' setting in the tool preferences). The eraser either puts down the background color, or turns and area transparent. clone and heal are very similar also both using a point of the image (set by Ctrl-clicking) and paints it onto another part of the image. That becomes very useful for removing parts of a picture or removing dust spots. The smear brush is also has an obvious functionality.