Big-bang is a theory given for birth of the universe. According to this theory all the mass in the universe was concentrated at one place, which exploded to give birth to all the stars, planets and other celestial bodies. The concentrated mass was extremely hot and dense. This lead to the explosion known as the big bang. The universe existed as a “singularity” before the big bang.
The Big Bang theory is an effort to explain what happened at the very beginning of our universe. Discoveries in astronomy and physics have shown beyond a reasonable doubt that our universe did in fact have a beginning. According to the standard theory, our universe sprang into existence as "singularity" around 13.7 billion years ago. What is a "singularity" and where does it come from? Well, to be honest, we don't know for sure. Singularities are zones which defy our current understanding of physics. They are thought to exist at the core of "black holes." Black holes are areas of intense gravitational pressure. The pressure is thought to be so intense that finite matter is actually squished into infinite density (a mathematical concept which truly boggles the mind). These zones of infinite density are called "singularities." Our universe is thought to have begun as an infinitesimally small, infinitely hot, infinitely dense, something - a singularity. Where did it come from? We don't know. Why did it appear? We don't know.
-well this is just a summary that i copied from my GK book hope it helps :D best of luck ...