It was We are the World.
It was written by Michael Jackson and Lionel Richie for the USA for Africa campaign. It was associated with the 'Hands Across America' movement, and was produced as a way for artists to help contribute to relief efforts of the famine in Ethiopia and other parts of the continent in during the mid 1980s.
The group raised over 100 million dollars from sales of the album in 1985. While 'We are the World' was written specifically for the album and very popular, another song was included in the album that was written for the effort by a group of predominantly Canadian artists. That song was called Tears are Not Enough.
The Canadian supergroup didn't just include musicians, but also included other notable Canadian born celebs. Folks like John Candy, Eugene Levy, and Catherine O'Hara. The American 'We are the World' supergroup had Dan Aykroyd! Check out the links to the wiki articles to see all the various stars in both groups.