It's essentially the story of five princes, the Pandavas, going to war against their cousins, the Kauravas. With the help of Krishna, a divine incarnation of the great Hindu god Vishnu, the Pandavas win the decisive battle at Kurushetra. It is a Sanskrit epic ascribed to the sage Vyasa. Besides the main story there are many small episodes, describing live of the ancient Indians and their practical solutions for difficult situations in life. The core piece of the Mahabharata is the Bhagavad Geeta, which has become the most famous Hindu scripture. One of the Pandava brothers, Arjuna, who is a master archer and is chosen to lead the Pandavas into battle against the Kauravas, is suddenly overcome by doubt, whether this war is really necessary. Krishna convinces Arjuna that it will be better for the future of mankind, even though it appears to be a cruel act at the time being. Arjuna understands that he can't escape his karma to exterminate the evil relatives. With Krishna as his charioteer and advisor Arjuna has the decisive advantage over his opponents.
In the eighties the story was made into a TV serial with 94 episodes, which you can find in YouTube. For the full text in English (transl. by Kisari Mohan Ganguli) as PDF file please click here: "The Mahabharata"