He helped bring about India's independence from British rule, inspiring other colonial peoples to work for their own independence and ultimately dismantle the British Empire and replace it with the Commonwealth. Gandhi's principle of satyagraha (Sanskrit: truth + path/way), often roughly translated as "way of truth", has inspired generations of democratic and anti-racist activists including Martin Luther King, Jr and Nelson Mandela. He often stated his values were simple: truth (satya), and non-violence (ahimsa).
Gandhi was born on October 2, 1869, in Gujarat, India. He was the son of a local official and trained as a lawyer in London. He went to South Africa to practise law in 1893 and began his political career by lobbying against laws discriminating against Indians in South Africa. Gandhi was arrested on November 6, 1913 while leading a march of Indian miners in South Africa.
Gandhi drew inspiration from the writings of Leo Tolstoy, who in the 1880s had undergone a profound conversion to a personal form of Christian anarchism. Gandhi translated Tolstoy's "Letter to a Hindu" which was written in 1908 in response to aggressive Indian nationalists, and the two corresponded until Tolstoy's death in 1910.
During World War I, Gandhi returned to India, where he campaigned for Indians to join the British Indian Army. After the war, he became involved with the Indian National Congress and the movement for independence. He gained worldwide publicity through his policy of civil disobedience and the use of fasting as a form of protest, and was repeatedly imprisoned by the British authorities (for example on March 18, 1922 he was sentenced to six years in prison for civil disobedience but served only 2 years). One of his most striking actions was the salt march that started on March 12, 1930 and ended on April 5, when he led thousands of people to the sea to collect their own salt rather than pay the salt tax. On May 8, 1933 Gandhi began a fast that would last 21 days to protest British oppression in India. In Bombay, on March 3, 1939 Gandhi fasted again in protest of the autocratic rule in India.
Gandhi became even more vocal in his demand for independence during World War II, drafting a resolution calling for the British to Quit India, which soon sparked the largest movement for Indian independence ever, with mass arrests and violence on an unprecedented scale. During this time, he even hinted an end for his otherwise unwavering support of non-violence, saying that the 'ordered anarchy' around him was 'worse than real anarchy'. He was then arrested in Bombay by British forces on August 9, 1942 and was held for two years.
Gandhi was a larger-than-life figure in relations between the Hindu and Muslim communities of India. It is said that he ended communal riots through his mere presence. Gandhi was vehemently opposed to any plan which partitioned India into two separate countries (as the plan which was eventually adopted did — creating a Hindu-dominated India, and a Muslim-dominated Pakistan). On the day of power transfer, Gandhi did not celebrate independence with the rest of India, but mourned partition alone in Calcutta instead. He was assassinated in New Delhi on January 30, 1948 by Naturam Godse, a Hindu radical who held him responsible for weakening the new government by insisting on a payment to Pakistan.
The most famous artistic depiction of his life is the film Gandhi, directed by Richard Attenborough and starring Ben Kingsley in the title role. Another film that deals with Gandhi's 21 years of life in South Africa is The Making of the Mahatma directed by Shyam Benegal and starring Rajit Kapur.
One curious fact often quoted about Gandhi's life was that he never recieved the Nobel Peace Prize though he was nominated five times for the same. The omission has been publicly regretted by the Nobel Committee years later. When the Dalai Lama was awarded the Peace Prize in 1989, the chairman of the committee said that this was "in part a tribute to the memory of Mahatma Gandhi". Though no opinions were forthcoming from the committe on the reasons for the omission, differing opinions point to various factors. Some observers say his struggle was "nationalistic" and "not a global one", while some others say the process of deciding the winning laureates were "not refined enough" then, to include non-European leaders. Some have even accused the committee of buckling to British pressure against the award to Gandhi.
The official Nobel e-museum has an article discussing the issue. 
Albert Einstein famously said of Gandhi, "Generations to come, it may be, will scarcely believe that such a one as this ever in flesh and blood walked upon this earth."