Read this tutorial untill the end - I garuantee that you will understand it after you read everything. If not, feel free to ask any question you might have.

Colors->Curves... (in gimp 2.2 tools->color tools->curves...)
curves dialog picture

So, What do we have in this picture?

A line like the one shown means that no adjustment will be made. In order to change this we click anywhere on the line and drag it.

The Bottom bar symbols the amount of something in the original image and the left bar symbols the new amount to adjust.
Black Symbols 0% and the more lighter it gets it gets a higher calue untill 100% in the white part.

In This exapmle you can see that I'm working on "Value" channel which means the brightness of the layer. Therfor "Black" symbols the Black parts (0% brightness) and White symbols the White parts (100% brightness).

since when we have a straight line from the Black Corner to the White Corner, the value of each point in original bar is matched with the same value in the "adjustment bar" (the horizonal value of the point as shown in the adjustment bar matches the vertical value as in the orifinal bar). The line represents the value of each pint.

Example:

Here I have clicked on the line and dragged it, to the top (the white part in the adjustment bar)
I'll notice 3 things:
1. Now each value from the "original values" has a lighter match on the "adjustment values" so we increased the brightness.
2. A new seperate bar apperas at the bottom and it represents the new value of this channel.
3. The layer will get Brighter


Now: We can enhance the contrast using the curves dialog:


Here We have decresed the values of the darker areas and increased the value of the lighter areas - this exactly how the contrast tool works - here you can enhance the contrast in any way you want

Now, when you understood how it works - let's go on to other channels.
Colors in GIMP are made out of RGB - Red Green and Blue.

Red+Blue=Magenta
Red+Green=Yellow
Blue+Green=Cyan
Red+Green+Blue=White

Decreasing the red will leave green and blue - Cyan
Decreasing the blue will leave green and red - Yellow
Decreasing the gree will leave blue and red - Magenta

Now: Let's Be practiacl:

Black on the adjustemnt bar is when you have 0 of the Red/Green/Blue.
So let's see what can we do:




There is also an alpha channel in the curves dialog - This is used to control the transpercy of the layer and it is handeled the same way as the other channels.

That's it - Hope you learned something :w:

If you want to read more - go to the GIMP documentation page that deals with the curves dialoog: http://docs.gimp.org/en/gimp-tool-curves.html (Thanks to photocomix)

EDIT: yey! apperantly this was officiated - my first offical tutorial =D

So that's how the curves dialog works. Thanks for the guide. =D

no problem

:h:well ...very clear i like the illustration too
i suggest add this link http://docs.gimp.org/en/gimp-tool-curves.html somewhere in the tut.

Thanks PhotoComix - also, I added your link.

~LIMN

I just caught two typos. A little under the first picture, you have "symbols" where you should have "symbolizes", and in the last line under the RGB picture, you have "Now: Let's Be practiacl:", where it should be "practical".