The Mahabharatha is the longest epic written in Sanskrit as well as the longest epic in the world. Along with the other major Hindu epic, the Ramayana, it is significant in the Hindu cultures of South and Southeast Asia. It is both a great work of literature as well as a source of devotion for Hindus. Since the Mahabharatha is complex and immense, I will break down some of the most important facts as an introduction. Maha means great and Bharatha refers to the king from whom it is believed the characters in the epic descended.
Who wrote it?: Veda Vyasa, a great Hindu sage who is said to have lived about 5,500 years ago, is credited with having written the Mahabharatha. However, he appears in the epic itself as a significant character who dictates the Mahabharatha to the deity Ganesha, who then writes it down with his broken elephant tusk. One version explains that Ganesha agreed to write the epic only if Veda Vyasa narrated the story without pausing.
When was it written?: The stories of the Mahabharatha existed in many different forms and versions for centuries. They circulated and were passed down through both oral and written traditions. The Mahabharatha was compiled as a single text only around 400 CE.
Main content: The backbone of the Mahabharatha is a narrative about the relationships and ultimate battle between the Kaurava and Pandava royal clans who are historically dated to about the 10th century BCE. The story is considered to be a combination of both the historical and mythic-- both a retelling of actual events and the insertion of mythical and spiritual anecdotes and interpretation. There are several smaller stories interspersed throughout the main narrative. (In this way, it differs from the Ramayana which does not contain so many tangential elements and is more straightforward in its narrative).
Other significant content: Along with the main narrative backbone, the Mahabharatha also contains several philosophical expositions. The Bhagavad Gita is the most significant of these. Literally translated "Song of the Lord", the Bhagavad Gita is the philosophical discourse delivered by Krishna (an incarnation of Vishnu and central figure in the narrative) to Arjuna (one of the five Pandava princes and the greatest archer). As the battle begins, Arjuna is overcome with grief by the fact that he must fight his own kinsmen. Krishna, who steers Arjuna's chariot, delivers this philosophical discourse, ultimately giving Arjuna the spiritual wisdom and insight to carry on with his righteous duty.
Cultural significance beyond the text: Even though the Mahabharatha is primarily considered a sacred text, its stories have manifested through other culturally significant mediums. For example, the stories have been expressed through the theatrical arts, dance and song. Most recently, the story has been adapted by film and television. Growing up, I came to learn about the main points of the story through a popular Indian television mini-series.
This introduction only provides a very rough sketch of the Mahabharatha. Much can and has been written about each story or narrative in the main plot in addition to countless commentaries, as the longest epic provides immense material for study and devotion for Hindus.