Short answers, pulled from the story.
Tennis traces its origins to 12th-century northern France, where players struck a ball with their palms. Modern lawn tennis was developed between 1859 and 1865 by Harry Gem and Augurio Perera in Birmingham, England, with Walter Clopton Wingfield patenting a version called sphairistike in December 1873 and popularizing it worldwide.
The four Grand Slam tournaments are the Australian Open, played on hard courts in Melbourne; the French Open, played on clay in Paris; Wimbledon, played on grass in London; and the US Open, played on hard courts in New York City. They are the most prestigious events in tennis and are the only tour events that last two weeks and include mixed doubles contests.
Among men, Novak Djokovic holds the all-time record with 24 Grand Slam singles titles, ahead of Rafael Nadal with 22 and Roger Federer with 20. Among women, Margaret Court leads with 24 titles, followed by Serena Williams with 23 and Steffi Graf with 22.
Tennis was an original Olympic sport but withdrew after the 1924 Games. It returned as a demonstration event for players 21 and under in 1984, with the reinstatement credited to ITF president Philippe Chatrier, ITF general secretary David Gray, and ITF vice president Pablo Llorens, supported by IOC president Juan Antonio Samaranch. Tennis became a full-medal sport again at the 1988 Seoul Olympics.
Hawk-Eye is an electronic review technology coupled with a point-challenge system that allows players to contest line calls during a match. It was adopted as a recent addition to professional tennis and uses camera tracking to determine whether a ball landed in or out.
The Open Era began in 1968, when the distinction between amateur and professional players was abandoned, allowing all players to compete in all tournaments including the major events. The change was driven by commercial pressures and reports of amateurs taking prize money covertly. It enabled top players to earn a living directly from tennis for the first time.