Playing with curves

[fstr21]

Does anyone have any suggestions TO DO's DO NOT's or general rules when working with curves, as of right now its really random to me and my general method is clicking a place on the window that makes the image look good, and that is about it.

[amica999]

Want to have a look here? http://www.lynda.com/home/DisplayCourse.aspx?lpk2=52105

[Der_W]

Curves are based on the luminosity of the individual channels.
So if you're clicking on a lower point, you'll most likely select a portion of the channel that is at this level of luminosity.
The rest on going up or down is really color theory.
In RGB lightening the individual channel's curves will add more of that color, darkening them will add the complementary colors (Cyan, Magenta, Yellow). Raising the composite will brighten/darken the image overall.
In CMYK its the same, the only difference is that you maybe have to raise the curves in the opposite direction (prepress folks find it easier to think in terms of density than in terms of luminance levels).
LAB is the only color mode where you can and most certainly will use very cricket curves in the individual channels (to be more precise: in the a- and b-channel).

Think of it this way: Curves are the far more complex version of levels. They allow you to modify one point and fix an other one to a certain position so it won't be modified as well. Levels just have the black-, white- and middle point for each channel.