By the orthodontist tightening your wires, adding rubber bands, etc. during your braces wearing, your teeth are either being pulled apart if too crowded, pushed together if too much space as well as correcting your bite by pulling teeth forward or pushing backward. I wore braces three times in my life. Didn't have all the choices they have today. Basic metal and wire.
The orthodontist glues metal brackets to the patients teeth. Then he places the wire which helps to fix whatever problems the patient may have. Simple right? The entire procedure for me took about 30 minutes and it was completely painless. If there are gaps, the wire will pull them closed. If there is crowding, the wire will pull them apart and align them. If they are just crooked the wire will make them straight. The braces will also fix overbites (the top teeth cover the bottom when biting down) under bites (the bottom teeth cover the top) and cross bites (a little bit of both). Overall, braces are really helpful to a person with bad teeth. They are not only for cosmetics. I have had them for 2 years now and they tend to get on my nerves but I love them :)