Short answers, pulled from the story.
Pink Floyd were formed in London in 1965 by Syd Barrett on guitar and vocals, Nick Mason on drums, Roger Waters on bass guitar and vocals, and Richard Wright on keyboards and vocals. David Gilmour joined as a fifth member in December 1967.
Syd Barrett left Pink Floyd in early 1968 due to deteriorating mental health. His condition, worsened by regular LSD use, made him unable to perform reliably; on several occasions he stood on stage with his arms hanging and guitar untouched. The band stopped collecting him for a show in Southampton, and Blackhill Enterprises announced his departure on the 6th of April 1968.
Pink Floyd have sold more than 250 million records worldwide, making them one of the best-selling music artists of all time. The Dark Side of the Moon alone has sold more than 45 million copies.
Roger Waters left Pink Floyd in 1985, writing to EMI and Columbia to announce his departure and going to the High Court to dissolve the band, which he declared a "spent force creatively." He later said the other band members had forced him out by threatening to sue him and that by not making new albums they would breach their recording contract.
The Dark Side of the Moon, released in March 1973, takes its title as an allusion to lunacy rather than astronomy. Waters wrote all the lyrics. The album is the world's third best-selling album, remained on the US Billboard Top LPs and Tape chart for more than fourteen years, and spent 364 weeks on the UK chart.
Waters reunited with Gilmour, Mason, and Wright for a single performance at Live 8 in Hyde Park, London, on the 2nd of July 2005, their first performance together in more than 24 years. Pink Floyd turned down a tour contract worth £136 million after the event. In 2022, Gilmour and Mason recorded the single "Hey, Hey, Rise Up!" as Pink Floyd without Waters.