Here is a site that has some information on the application of the "Curves" function in Photoshop.

I believe that this could include and extend to other image editing programs.

Easy to follow and yes, quite basic ... but, for the novice to the program and digital editing and enhancements in general, it may be of some help. :o)

Rickmann Design. (This link will take you directly to the page on the site discussing the "Curves" function, complete with screenshots taken from Adobe's Photoshop program.)