Posture is the way you position your body or arrange your limbs. So stand up straight, put your shoulders back, and lift that chin up.
Ballerinas have an elegant, graceful posture, and soldiers tend to display a rigid, strong posture. If you sit hunched over your desk every day you'll end up with terrible posture. This noun describes the way you carry yourself, but can also describe a certain body position you take, like the different postures in yoga. If you're accused of posturing though, that's a different story — that means you're behaving unnaturally to impress someone.