Modern

Modern is a dance style that rejects many of the strict rules of classical ballet, focusing instead on the expression/emotion of the dancer's feelings.  It encourages the dancer to use their emotions and moods to design their own steps and routines.

Martha Graham and Isadora Duncan are two of the many influential choreographers who have helped in the development of modern dance from the past 150 years.  Martha Graham used releasing techniques where she rejected the primary importance of ballet positions, and did her own movements where the source of energy was the center of the body.  Isadora Duncan used ancient Greek for her inspiration of dancing where she learned to project vivid personalities through music, motion and stillness of self-expression and passion.

Modern dancers often use their body weight to enhance their movements, whereas classical ballet dancers strive to be light and airy of their feet.

Reference:
-Susan Cooper. Staging Dance. London: A & C Black Limited, 1998.
-http://www.wisegeek.com/what-is-modern-dance.htm