GIMP has for main tool groups, selection tools (A), paint tools (C), transform tools (B), and color tools (which must be accessed from the tool menu). This page will outline the uses for some of them and show some of the out puts.
Selection tools
If you are doing work with layers then these will come in handy.
The rectangle 
and eclipse 
select tools do what you think they would, just make the shape you want and press enter or click in the middle, now you have a selection that you can many things with, but an easy one is copy and paste.
and eclipse select tools do what you think they would, just make the shape you want and press enter or click in the middle, now you have a selection that you can many things with, but an easy one is copy and paste.
to move it to where you want it. I will get into the move tool later but it is very useful.
Use fuzzy select to select an area of an Image that is a consistent color, it will select all the pixels that are connected and have a difference of less than a user value, by default 15. 
The select by color tool works almost the same as the Fuzzy tool however; it will select all the pixels on the Image that have a difference of less than 15. Watch that it does not have a few pixels selected where you do not want them, like in the image on the right in which I was trying to select the sky. All was well untill the house was selected in the indicated area.
