The neck and lumbar sections of the spine have a lordotic curve, which means that they curve inward whereas the thoracic spine has a kyphotic curve, meaning it curves outward. These curves help maintain upright balance whilst supporting the weight of the head and upper body. However, too much curvature may cause spinal imbalance leading to spinal conditions that may result in pain or loss of mobility.
Typical adult spines are positioned above the pelvis, so upright posture doesn't strain muscles. However, changes in spinal position can cause undue stress to muscles and lead to spinal deformity. If the spine is injured and unable to function properly, it can be very painful and in some circumstances, disabling.
The spine is usually a straight line when viewed from the back with some normal, gradual curves when viewed from the side, however if the spine curves to one side or the other, this indicates a condition named scoliosis.