Just like eyeshadow, eyeliner can change the shape of your eye. It's really just down to playing around with different styles and different types of liners, until you find some you like. Here are some examples:
Tight-lining is when you apply eyeliner to the upper water line, and in between the lashes right at the lash line. Here, I've show the difference that it can make. I tight-lined using pencil eyeliner. All pictures are without mascara.