Monsoon winds are just like land and sea breezes caused by differential
heating during summer and winter, except that they are on a large continental scale instead of just being to the coastal regions. They are most prevalent in countries in Asia especially the sub- continents of India, China, Japan and Southeast Asia.
During summer in the Northern Hemisphere the interior Asia is much hotter the sea around. Rapid heating induces the warm air to rise, creating intense low pressure. At the same time in the Southern Hemisphere, which is, experiencing winter, the low temperatures and cool air creates a region of high pressure. Winds are thus sucked across the equator into the Indian Ocean and hen the Indian sub-continent, bringing heavy rain in the form of the South-West Monsoon.
The converse
takes place in the winter. The interior of Asia in the Northern Hemisphere is much colder than the sea around it. Rapid cooling and denser air create a region of high pressure. Meanwhile in the Southern Hemisphere it is summer and the warm air rises creating a region of low pressure. Winds thus blow out of the continent of Asia into the Indian Ocean as the North-East Monsoon
http://answers.yahoo.com/question/index?qid=20081114072543AAGtpGx