Star clusters are collections of stars held together by their mutual gravitational attraction. The orbit of the stars within them are dominated by close interactions with other stars. This distinguishes them from galaxies, in which the orbits of stars, such as the Sun, are governed by the overall smooth potential of the very distant stars and dark matter.  Also, there is no evidence for the presence of dark matter in star clusters. Clusters are extremely interesting objects to study from a dynamical point-of-view. The topic of dynamical evolution of star clusters is the classical N-body problem: finding the motion of N points of given masses, mutually attracting themselves as the inverse square of their distance.