Broadly speaking, the "Great Rebellion" (known at the time as the "Sepoy Rebellion" because it was led by "sepoys," or Bengal soldiers serving under the British) was caused by the sense that the British were exploiting the Indian people and threatening to expunge traditional Indian culture. There were many other factors, such as forced commercialization of farming, taxation, and others, but these cultural concerns were profoundly important.
The British disrespect for the people they ruled was symbolized by a story that spread through the ranks of the Bengal army. The story held that cartridges to be used in new rifles issued to Bengal soldiers were greased with pork fat, which was anathema to Muslims, or beef fat, which Hindus could not consume for similar reasons.
Rebellions broke out in the ranks of the sepoys, and quickly spread throughout Northern India. As for the legacy of the rebellion, its most important legacy was that the British crown took control of the subcontinent from the East India Company. The new government attempted to create loyalty and stability by building infrastructure, respecting traditional Indian culture, and simultaneously educating a class of Indian people to serve in the imperial bureaucracy. Indian princes were granted increased autonomy, and the commercialization of the Indian economy continued, and even accelerated.