What was the Sandpapergate scandal that rocked Australian cricket in 2018?

During the 2018 Cape Town Test against South Africa, Australia's Cameron Bancroft was filmed rubbing the ball with sandpaper to roughen one side to gain unfair advantage. Cricket Australia found that David Warner devised the plan and captain Steve Smith failed to prevent it. Smith and Warner received 12-month bans, while Bancroft was suspended for nine months.