The city of Johannesburg (Joburg, Jozi, Egoli) is the largest in South Africa, and the greater Joburg metropolitan area is one of the 50 largest in the world. Depending on how you define it—as just the city proper or including the broader area—its population is between 4.5 and 8.5 million. It’s also one of the world’s ‘greenest’ cities, with some 6 million trees.
If you’d like to do some siteseeing while you’re here, Joburg Tourism has a good guide to a variety of options—and the Guardian has a useful alternative city guide to Johannesburg. Johannesburg In Your Pocket is a great, comprehensive resource, and the nothing to do in joburg besides… blog is definitely worth checking out.