I am going to try and take quite a few concepts and totally simplify them to keep this post short, so bare with me.
There are 2 types of muscles: Mobilizers: close to body surface, “fast twitch” fibers, produce power, but lack endurance. Use them and they get tighter and shorter and assist you with rapid movement. Stabilizers: deeper in your body, “slow twitch” fibers for endurance, but become weak and long over time. These are what assist with your posture. What is a muscle imbalance? This comes down to the relationship between the “tone” or strength and the length of muscles around joints. Over time your mobilizers can inhibit stabilizers and even try to steal their job, which causes imbalance. Imbalance can be seen by looking at a person’s posture. Why is excellent posture important? Great posture helps you to fatigue less easily, provides shock absorption, puts less stress on joints, transfers energy during movements more effeciently, help you to avoid overtraining, and will determine which muscles are strong/weak by lengthening or shortening certain muscles (good stuff, eh!). Plus, you look smarter with better posture! Postural misalignments: Some posture issues are structural (meaning scoliosis, fused vertebrae, replaced joints, etc) and can not be fixed with stretching and/or exercise. However, many folks misalignments are functional (meaning they are caused by muscle imbalance). How to tell if your posture stinks: Look at the frontal plan (side profile). There should be a vertical line from your ears to shoulders to hips to knees to heels/ankles. Then look at the sagital plane (face forward) and your shoulders should be in line with each other, as your right and left hip and your right and left knee. Your feet should not pronate or supinate (tip in or outward). Think of the anatomical position. You know, the hanging skeleton in the corner in science class.