The Mahabharata is a collection of many different stories around one central story. The central story is this:
The five good Pandava brothers fight with their cousins, the hundred bad Kaurava brothers, to see who will rule the cities of the upper Ganges river and the Jumna river (in north-western India). And in the end, with the help of many Hindu gods and especially Krishna, the Pandavas win.
One little piece of the Mahabharata, with English subtitles
One of the brothers comes home victorious
But the story has thousands of pages, and tells many other stories about the characters. One part is an early version of the Cinderella story. Another important part of the Mahabharata is the Bhagavad Gita, which is a kind of speech about how to be good that has had a lot of influence on Indian thought.
The Mahabharata more or less tells the story of Aryan conquests in India around 1000-800 BC, and it was probably first composed around 500 BC and then written down around 300 BC, in Sanskrit. In the Mahabharata, the gods are always on the side of the Aryan Pandava brothers, and never help the bad Kauravas. The bad guys, in fact, are often represented as snake-people.
(Compare the Greek story of Perseus).
This is the Aryan view of things - the pre-Aryan, Harappan or Dravidian people who lived in India would have taken a very different view of who was good and who was bad. The Aryans, after all, were invading the Dravidians' country and taking it over. But the Aryans won the war, and so their version of the story also won.
To find out more about the Mahabharata, check out these books from Amazon or from your local library:
Savitri: A Tale of Ancient India, by Aaron Shepard (1992). A retelling for kids of one of the more feminist stories in the Mahabharata, with lots of pictures in a traditional Indian style.