A leak is information about a game that has been released prematurely.
In Super Smash Bros. Brawl one of the most famous leaks was the Ness, Lucario, and Jigglypuff leak. Nintendo's Japanese website posted a video of various features in Super Smash Bros. Brawl, including how the use of stickers works. At around 4:21 in the video, various stickers are shown and in the bottom part of the screen is the head of characters that could equip the sticker. Ness is next to Lucas for one of these stickers, while Jigglypuff and Lucario appear next to Pikachu and the Pokémon Trainer in another. Because the heads were used to represent playable characters, this confirmed that Ness, and Jigglypuff would be returning in Brawl, and that Lucario would be making his first appearance into the franchise.
There two leaks, the Gematsu Leak and the Bowser Jr. Leak (Bowser Jr., Shulk, Ganondorf, Duck Hunt Dog, etc.). The Gematsu Leak is believed to be fake after the problem of Chrom being part of Robin's Final Smash. The other part of this disaproval is the reveal of Rosalina. The latest leak says Bowser Jr., Duck Hunt Dog, Shulk, Ganondorf, Shulk (revealed), Ness, Mr. Game and Watch, R.O.B., and Wario are confirmed. There are leaks floating around the internet, even actual footage, these can be found on NEOGAF.