You know how once you figure out the best style of jeans for your body, that everything just looks better? It's kind of the same concept for your face.
Learning your face shape will help you determine what haircuts are most flattering, and how to make up for what you may think are "problem areas". Ultimately, your hairstylist should be able to tell you what your face shape is and help you choose a hairstyle to fit it.
The simplest way to determine your face shape is to pull your hair back in a tight ponytail, look straight into the mirror, and trace the shape with a dry-erase marker (or something you can wash off the mirror). Step back, and figure out what shape it is.
I like to be flexible when it comes to sticking to "beauty rules" so I hope you interpret these suggestions as options and not the only haircut you should have. Texture, layering, and color can all add dimension to hair and help a certain cut or shape work with any face shape.