A former employee of Cambridge Analytica, Christopher Wylie, blew the whistle on the data analytics company improperly obtaining a trove of Facebook user data. The New York Times and The Guardian broke the story, reporting that 50 million Facebook users' data had been leaked; the number is actually closer to 87 million users.
The New York Times characterized Cambridge Analytica as a "voter-profiling company" that promised it had "tools that could identify the personalities of American votes and influence their behavior" — it used Facebook user data to perfect those tools, which ultimately were used in Trump's successful 2016 presidential campaign.
Cambridge Analytica got this user data from personality quizzes offering OCEAN (openness, conscientiousness, extroversion, agreeableness, neuroticism) tests to Facebook users.
In April, Zuckerberg testified before Congress about the scandal, as well as Russian influence campaigns and allegations of anti-conservative bias.
Sources: Wired, Wired, New York Times, The Guardian, Vox, Business Insider