The Tiananmen Square protests of 1989 were protests in April and June of that year. China calls them the June Fourth Incident, but to most of the world it is the Tiananmen Square massacre.
There were other protests on Tiananmen square in 1919 and 1976. The protests of 1989 were organised by groups of students, intellectuals and labour activists. There was no common cause or leadership in the protests. However, most protesters did not like the way the Communist party of China ran the economy. Some people also wanted a change towards more democracy. Most people protested on Tianmen Square in Beijing, but some also did in other cities, like Shanghai. The protests in cities other than Beijing stayed peaceful.

The official Chinese position on the events was that the protests needed to be dispersed in order not to harm the stability of the country.
The protests had various causes. The corruption of the Communist Party was a main issue. The control of the economy was working badly. The country was poor, and city dwellers got the worst of it.
Ironically, this all happened under the rule of Deng Xiaoping. He was Mao Zedong’s successor in 1978 and launched the economic reforms. These reforms helped life for farmers and people who lived in the countryside, but was bad for city dwellers. Intellectuals, students, and industrial workers, feared unemployment rising and social problems. They felt disenfranchised. This means people felt they could do nothing about these problems.
Many of the scenes were shown around the world over western media like the one below. The most famous image is this picture showing a man standing in front of a file of tanks in the middle of the square.
Links to some pictures:
Tiananmen Square occupied by tanks
Protestors in the Square
A Type 59 main battle tank on display at the Military Museum of the Chinese People’s Revolution in western Beijing. On June 3, 1989, People’s Liberation Army deployed Type 59 tanks in the crackdown.