Posture is the position in which you hold your body upright against gravity while standing, sitting, or lying down. Good posture involves training your body to stand, walk, sit and lie in positions where the least strain is placed on supporting muscles and ligaments.
When singing, you should always 'stand from the waist up.' If sitting, you should be on the front edge of the chair, or as our chorus director says, "Buns on the front burner." If standing, you should relax your muscles, but still be straight enough so that you do not crush your diaphragm.
Picture taken by Caitlin
For more information about posture, visit this site: http://www.clevelandclinic.org/health/health-info/docs/0300/0359.asp?index=4485