Wiki says it as a “The initial cause of the conflict was a disputed region of the Himalayan border in Arunachal Pradesh, known in China as South Tibet. Fighting began on 20 October 1962 between the People's Liberation Army and the Military of India.”
Result: Chinese military victory. Subsequent Chinese withdrawal to pre-war positions
“The aftermath of the war saw sweeping changes in the Indian military to prepare it for similar conflicts in the future, and placed pressure on Indian prime minister Jawaharlal Nehru, who was seen as responsible for failing to anticipate the Chinese invasion.”
Very interesting note is presented on why china went to war with India . 1) There was "a perceived need to punish and end perceived Indian efforts to undermine Chinese control of Tibet, Indian efforts which were perceived as having the objective of restoring the pre-1949 status quo ante of Tibet" The other was "a perceived need to punish and end perceived Indian aggression against Chinese territory along the border.
2) Another factor which affected China's decision for war with India was a perceived need to stop a Soviet-US-India encirclement and isolation of China. India's relations with the Soviet Union and United States were both strong at this time, but the Soviets were preoccupied by the Cuban Missile Crisis and would not interfere with the Sino-Indian War. P.B. Sinha suggests that China timed the war exactly in parallel with American actions so as to avoid any chance of American or Soviet involvement. American buildup of forces around Cuba occurred on the same day as the first major clash at Dhola while China's buildup between the 10th and 20th of October coincided exactly with the United States establishment of a blockade against Cuba which began on the 20th of October.
Another link on why Arunachal Pradesh’s claim by china. It’s a very interesting read.
“The major cause of China-India territorial dispute, which deprived peace from Arunachal Pradesh, is from immediately after Chinese Revolution in 1911. At that time, Qing Dynasty, the last Chinese Dynasty, fell and the Emperor of Qing Dynasty resigned. As a result, China, Tibet and Mongolia became equal and separate countries. In these three countries, Tibet and Mongol tried to be approved internationally as independent states, and undertook to international society. However, on the other hand, Han Chinese Republicans started small war to subjugate reins of government of Tibet and Mongol. Therefore, they could not be independent states at that time.
In 1912-13, the British Raj made agreements with tribal leaders that defined the area that was known as NEFA. British rule could have very small impact on the isolated, self-sufficient in terms of economy and militarily uncontrollable tribal peoples. In order to arrange this conflict, in 1913 and 1914, the British administrator, Sir Henry McMahon, drew up the 550-mile McMahon Line as the border between India and China in Shimla, during a conference which also discussed the Tibetan and Chinese borders (“McMahon Line.”) This conference is called Shimla Conference. Regardless of this reconciliation, the McMahon Line was rejected by the Chinese government in 1947 (“Arunachal Pradesh.”)
Following this dispute, the Chinese troops crossed the McMahon Line on the August 26th, 1959 (“Arunachal Pradesh.”) They abandoned this in 1961, but in October 1962 crossed the line once again with force (“McMahon Line.”) As a result, Sino-Indian War broke out. The Chinese extended their frontier as they intruded deeper and deeper. However, this war did not go on for a long time. In 1963, the Chinese agreed to withdraw back to the McMahon Line, and also returned Indian prisoners of this war at that time.
Another interesting read, stand on India and china by world in general
PS: This is just a meagre attempt to search/learn on the mentioned topic, explicit views wud referred to google