Why does India have a monsoon climate and what causes the rains to reverse?

The word monsoon comes from the Arabic 'mausim', meaning season, and refers to winds that reverse direction seasonally. The reason is the uneven heating of land and sea. In summer, the land mass of India heats up faster than the Indian Ocean, creating a low-pressure area over land. Winds blow from the high-pressure sea towards the low-pressure land, bringing the moisture-laden South-West Monsoon and heavy rain from June to September. In winter, the land cools faster than the sea, so pressure conditions reverse: now winds blow from land to sea as the dry North-East Monsoon. That is why most of India gets its rain in summer. This seasonal reversal of winds, driven by differential heating of land and water, is what defines India's monsoon climate.