The Big Bang is a model that describes how the universe started. The current estimate for The Big Bang is 13,730,000,000 years ago (13.73 billion). The theory is not a finished theory and is still at the cutting edge of astro-physics. The current Big Bang theory is very likely to be modified in the future. Some of the ideas are extremely abstract and complex and can make your head spin if you are not careful!
But very basically, the universe started as an almost infinitely dense, infinitely hot, infinitely tiny point and 'exploded' out in all directions at the speed of light (it is still expanding today because of that initial explosion).
The Big Bang Theory replaced the idea that the universe was eternal and had always existed. It has been theorised that there may have been many other Big Bangs and many other universes.
It is possible that our universe will eventually stop expanding, (caused by gravity pulling the universe back together) and it will start to shrink and pull itself back to an almost infinitely dense, infinitely hot, infinitely tiny point and 'exploded' again!
Big Bang is the name given to an event which, scientists believe, occurred about 10 to 20 billions years back resulting in formation of this universe with all its galaxies and stars, including our own earth, sun, moon and the planets.
As per the Big Bang Theory the originated wit a big explosion like occurrence. This explosion gave rise to very strong radiation. These radiations formed an expanding region called primordial fireball. As this fireball cooled different types of matter formed.
This matter broke into big pieces and separated forming the galaxies, which in turn split to form the stars. Our own sun is one such star. The stars split further to form planets rotating around the star. Our earth is on such planet. And our moon is a satellite of earth, formed by separating from it.