The iconic image of that event is the photo of a single unidentified man holding two shopping bags who steps in defiance of a line of army tanks heading to Tienanmen Square.
All through the months of April and May 1989 Chinese students and pro-democracy activists protested in Beijing and other parts of the country. The protests were triggered by the death of Hu Yaobang, a liberal reformer, who was deposed after a power struggle with hardliners over the direction of political and economic reform in China.
On the nights of the 3rd and 4th June 1989 the Chinese Government enforced martial law.
The authorities proceeded to carry out mass arrests. Some people were executed and others just disappeared. An estimated 1600 were imprisoned for long periods, and 2 remain in prison 25 years later. Others were politically punished and lost jobs and positions. Many of the student leaders were forced to flee China.
Various slide shows of the 1989 events at Tienanmen Square are here, here and here.
Amnesty has recorded the names of Chinese activists targeted by the Chinese authorities in the lead up to the anniversary. This includes scores of detentions and house arrest and people being charged with offences carrying prison terms for holding private memorial gatherings for the dead.
The New York Review of Books has this interview with Hu Jia, who was just 15 at the time he participated in the Tienanmen Square demonstrations. He is currently under house arrest in China over his attempt to organize a commemoration of the massacre protest activities.