The Universe is defined to be the sum total of everything that we know about: stars, galaxies, clusters, superclusters, voids, etc.
Cosmology is the study of the structure and evolution of the Universe.
It is a subject of great current interest, as the Hubble Space Telescope and other new instruments have recently helped astronomers shed light on many important questions.
There have been many theories about the Universe, but they generally all share one fundamental postulate, the cosmological principle, which has two parts. First, the Universe must be isotropic (the same in every direction).
For example, the Hubble deep field images, extending 12 Gly away, for two completely different directions, are remarkably similar:
