Updated: Jan 18, 2021
Starting in March of 2016, members of the Russian GRU (a foreign intelligence agency) hacked into the email accounts of the Clinton Presidential Campaign, the DNC, and the DCCC. They then worked with WikiLeaks to release the information at prime times before the 2016 election date (October 2016) with the email leak of her campaign chairman, John Podesta, being the most notable. The Trump campaign then utilized this hacked and leaked information for their own electoral benefit.
How was the information released? The blogs Guccifer 2.0 & DC Leaks, which were run by Russian GRU agents under the guise of American hacktivists, and WikiLeaks spread the leaked information to different media outlets such as HelloFLA!, a Florida focused political blog; The Smoking Gun; Sputnik & RT, both Russian based media outlets; Infowars; and others. Even key figures like Roger Stone, a lobbyist who did not officially part of the 2016 Trump campaign but was a close companion of Donald Trump and other members of the Trump Presidential Campaign, spoke directly about WikiLeaks and the contents of the hacks to his right-leaning niche audience.
These leaks were incredibly strategic, lining up with different major political events during the 2016 campaign year, in order to maximize impact. For example, one major leak of information occurred on the eve of the Democratic National Convention whereas another was released immediately following the publication of the Donald Trump Access Hollywood tape.