The Marvel Comics Universe has no shortage of massively-powerful beings who could easily defeat the vast majority of Earth's heroes; these characters occupy a space within the canon known as Marvel Cosmic, and Galactus is the best-known entity by far. Also known as the Devourer of Worlds, Galactus destroys planets without remorse as he travels through the universe. Yet for all his power, Galactus is a veritable ant compared to The Living Tribunal, who is essentially the right-hand man of the Marvel Universe's God.
The Living Tribunal first appeared in a classic Doctor Strange tale in Strange Tales #158. When Strange unleashed the monster Zom, he incurred the wrath of the Tribunal, who judged that Earth must be destroyed. For all his power, Strange had absolutely no chance of defeating his enemy in a fight, and thus asked the Tribunal for more time to prove that Earth was worth saving. Strange prevailed, but barely, and the Tribunal disappeared - though his past and powers would be fleshed out considerably after the story concluded.
The Living Tribunal was created at the dawn of existence and protects the Multiverse from any threat that might end or severely damage space and time. There are no variants of the Tribunal; much like the other powerful forces of the cosmos, it is the only one of its kind in existence. The Tribunal protects the balance of power in the Multiverse, and no one entity can have too much or too little. While Galactus can eat planets, the Living Tribunal can wink them out of existence - along with their entire solar system - in a second if he so chooses. The three heads of the Tribunal each represent Equity, Necessity, and Revenge - and unless all three are in agreement, the Tribunal will not take action.
The Living Tribunal will not act unless the multiverse is at terrible risk or an individual/cosmic force has become too power. During the Infinity Gauntlet, he didn't act upon Thanos acquiring the Infinity Stones, since Thanos had acquired them with his own strength and intelligence instead of "cheating" the cosmos (and only one universe was at stake, not the entirety of existence). The character rarely appears, and a sighting of the Tribunal exerting his full power - as seen in 1996's Marvel vs. DC, for example - is a sign that something has gone terribly wrong.
The Living Tribunal is only the second most-powerful character in the Marvel Universe. The One Above All reigns supreme and is essentially Marvel's equivalent to God (and once took the form of Jack Kirby in a Fantastic Four comic). While Galactus and his ilk may believe themselves to be superior, The Living Tribunal is a living reminder that there's always a bigger fish in the sea.