Collections of motile organisms such as flocking birds, schooling fish, and swarming bacteria often exhibit collective motion with extended spatiotemporal coherence. While there are many qualitative observations, quantitative measurements of such collective motion have been limited. We measure simultaneously the positions, velocities, and orientations of up to a thousand bacteria in a colony. The motile bacteria form closely-packed dynamic clusters within which they move cooperatively. The number of bacteria in a cluster exhibits a power-law distribution truncated by an exponential tail. Mobile clusters cause anomalous fluctuations in bacterial density, i.e., fluctuations N which vary with the mean N, not as N  N1/2 (as in a system in thermal equilibrium), but as N  N3/4.