The principles are based on trying to balance the colours.
In a 'levels' screen the left hand side controls the dark side of the image (whatever channel, RGB or as a whole), the right hand side covers the highlights (again whatever channel), the slider in the middle covers the brightness of the image.
If you select each channel in turn you can play with the balance of the colours.
http://www.cambridgeincolour.com/tutorials/levels.htm is a good tutorial on using levels.
The use of the droppers is very useful and is worth reading up (detailed at the end of the above article).