Broadly, the Mahabharata tells of the rivalry and war between the Pandavas and Kauravas. The Pandavas (Yudhishtira, Bhima, Arjuna, Nakula and Sahadeva) were the five sons of King Pandu, while the Kauravas were their 100 cousins, the sons of King Dhritarashtra. Aiding the Pandavas is Krishna, an incarnation of the god Vishnu.
The Mahabharata includes many other tales as well. A notable section is the Bhagavad Gita, a philosophical discourse between Arjuna and Krishna.