The Mahabharata is one of the two major Sanskrit epics of ancient India, the other being the Ramayana. In the much the same way that The Illiad is the national epic of Greece, The Mahabharata is the national epic of India. With more than 74,000 verses, long prose passages, and some 1.8 million words in total, it is the longest epic poem in the world.
It is also regarded as the great repository of  Hindu mythology. There are great creation stories – Manu’s flood, the descent of the Ganges, stories of Ganesh and Krishna. The epic also contains the codes of law - moral, ethical, natural.
Few people have the time to read all of this epic. And even fewer people can speak Sanskrit, so experiencing the original poetry will be filtered by translation. The Great Books List suggests the excellent shortened prose version by RK Narayan if you want to experience the story, although you will miss out on the poetry. It might be worth tracking down some verses online to experience the rich detail. Portions of the Mahabharata, The Ramayana and the Bhagavad Gita are include in this list as separate works.
The poem is of immense religious and philosophical importance in India and is a major text of Hinduism. Its discusses of all aspects of human strife -- wealth (artha), pleasure (kama), duty (dharma), and liberation from the cycle of birth and rebirth (moksha). The story also explains the relationship of the individual to society and the world  and the workings of karma.
Traditionally, the Mahabharata is attributed to the poet Krishna Dwaipayana Vyasa, although its immense length and composition layers suggests multiple authors over a period of time. The final form we know today was completed by the first century, although elements of its central core dates back to the 6th century BCE.
The Mahabharata includes many stories of Hindu mythology, cosmological stories of the gods and goddesses, and philosophical parables aimed at students of Hindu philosophy. Some of the most famous stories include in the poem are the Bhagavad Gita. Damayanti (a love story),  The Krishnavatara (the story of Krishna,) and an abbreviated version of the Ramayana saga.
On one level the story is merely the story of the Pandava dynasty, but it is also a grand discussion on the meanings of life, death, honour, bravery, morality and the true nature of the universe.

The Mahabharata
The core story of the work is that of a dynastic struggle for the throne of Hastinapura, the kingdom ruled by the Kuru clan. The two branches of the family that participate in the struggle are the Kauravas, the elder branch of the family, and the Pandavas, the younger branch. Calling the epic simply a dynastic struggle however is like calling Hamlet a story about a crazy Danish prince. The story is rich in episodes and detailed description.
The struggle culminates in the Great battle of Kurukshetra, in which the Pandavas are ultimately victorious. The Mahabharata itself ends with the death of Krishna, and the subsequent end of his dynasty, and ascent of the Pandava brothers to Heaven. It also marks the beginning of the Hindu age of Kali, the fourth and final age of mankind, where the great values and noble ideas have crumbled, and man is speedily heading toward the complete dissolution of right action, morality and virtue.