Ragnarok is a legendary event in Norse mythology. It is known as Gotterdammerung, "Twilight of the Gods", "Destruction of the Powers" or "Doom of the Gods". It's a predestined death of the Gods and end of the world.
It is said that first Fimbulvetr, the winter of winters, will occur and after such three more winters will happen with no summer inbetween them. Skoll, a mythological wolf figure, will finally catch the sun and devour her whole. Hati, another mythological wolf figure, will catch the moon and either strangle/devour him. As the story goes, this plunges the world into darkness: no more stars, no light.
Cocks will begin crowing and raising the dead. Fjalar will crow to the Giants, Gullinkambi, a golden cock, will crow to the Gods and a third will raise the rest of the dead. Jormungand will then begin making his way to land. As this is happening a ship called Naglfar will set sail, as Jormungand's movements will cause waves to free it. It is said to be made from the nails of dead men and will be packed to the brim with Giants who are led by Hymir.Loki, free from his bonds, will set sail from Hel with the dead. As this happens, the sons of Muspell will begin to move south. They will be led by a Flame Giant named Surtr. Heimdall will sound his horn- alerting the Gods. Eventually, everyone will converge on Vigrid- a large battle ground.
From there it gets a bit confusing: many stories depecting various Gods fighting and defeating each other. It is said that Odin will fight with Fenrir, and Thor will fight Jormungand. Thor will kill Jormungand, but being the posionous serpent that he is, Jormungand will bite him and the posion will kill the thunder God. Surt will attack and defeat Freyr, as he will be defenseless without sword. The hound, Garm, will attack one-handed Tyr and the two will kill each other. As this happens, it is said Loki and Heimdall will meet in battle but neither will survive. Odin will be devoured by Fenrir. However, Fenrir's victory will be short lived as it is said that Odin's son, Vidar, will grip Fenrir by the jaws and rip him apart.
Surt, the previously mentioned fire giant, will begin to sling fire everywhere with his sword after the defeat of Freyr. This will set fire to the World Tree, Earth, and eventually all nine worlds will burn and sink down into the depths of the sea.
However, that's not the end of it. Some stories say that a new ideal world will come up from the ocean, abundent in resources, green and vast. Some Gods will be reborn, others will have survived and come back. Lif and Lifthrasir, two humans that had been hiding in Yggdrasil, will see a new light rise: even though the Sun had been devoured, she had birthed a daughter who could fulfill her position. The descendants of these two will rule the Earth in peace and happiness, so it says. The Gods will begin to rebuild.
That is more or less the story. There are several more details, but if you'd like to hear them I'd suggest you look it up.