Short answers, pulled from the story.
The name Johannesburg was officially used for the first time on the 3rd of October 1886. Surveyor Jos de Villiers mapped the initial neighborhood called Randjeslaagte between mid-October and early November of that same year.
Jan Gerritse Bantjes discovered the main Witwatersrand gold reef on the farm Vogelstruisfontein in June 1884. This find triggered a massive migration of prospectors to the area that would become Johannesburg.
Black Africans account for 73% of the current population while whites make up only 18%. The apartheid government began constructing townships for black families outside Johannesburg in the 1950s to house these populations.
Johannesburg produces 16% of South Africa's gross domestic product and accounts for 40% of Gauteng's economic activity. The city hosts Africa's largest stock exchange called the Johannesburg Stock Exchange though it has moved out of the central business district.
About 575 people mostly black were killed during the Soweto uprising of 1976 which began on the 16th of June 1976. Demonstrations broke out over a government decree requiring black school-children to be educated in Afrikaans instead of English.