The term line in ballet refers to the configuration of the dancer’s body, whether in motion or at rest. Good line is partly a matter of the physique a dancer is born with, but it can also be developed and enhanced by training. In ballet, certain relationships of the arms, legs, head, and torso are considered particularly harmonious, while others are not, although they may be perfectly acceptable in different forms of dance. Large movements of the whole limb are preferred to small, isolated movements of individual body parts. Ballet is often described in terms of moving upward and outward; ideally, the dancer’s limbs should appear to extend into infinity.

— stolen from http://www.ccs.neu.edu/home/yiannis/dance/ballet.html