What is East Pakistan and West Pakistan?

In 1947 British India was partitioned along religious lines, any areas where the Hindus were in majority became India. Pakistan was created as a homeland for the Muslims. What became East and West Pakistan were the areas of British India that had a majority Muslim population. East and West Pakistan were situated on different ends of the subcontinent with India in the middle.
West Pakistan is modern day Pakistan and East Pakistan became Bangladesh in 1971.