Short answers, pulled from the story.
Ymir is the primeval ancestor of all jötnar (giants) in Norse mythology. He was formed from drops of poisonous venom that dripped from the icy rivers called the Élivágar into the void of Ginnungagap, and he is attested in the Poetic Edda, the Prose Edda, and the poetry of skalds.
Ymir is also called Aurgelmir, Brimir, and Bláinn in the Norse sources. Aurgelmir is his name among the jötnar; Brimir means "the bloody moisture" in Old Norse; and Bláinn's meaning is disputed.
Ymir was killed by the three brother gods Odin, Vili, and Vé, the grandsons of Búri. His wounds produced so much blood that all the jötnar drowned except Bergelmir and his wife, who survived on a lúðr and repopulated the giant race.
Odin, Vili, and Vé carried Ymir's body to the middle of Ginnungagap and fashioned the world from it. His flesh became the earth, his blood the seas and lakes, his bones the rocks, his hair the trees, his brain the clouds, and his skull the sky. His eyebrows formed Midgard, the realm of humans.
Scholars D.Q. Adams and J.P. Mallory identify Ymir as a central example of a reconstructed Proto-Indo-European cosmogonic myth in which a primordial being is dismembered and the universe is created from its parts. The pattern appears in the Old Indic Purusha sukta, the Middle Persian Škend Gumānīg Wizār, Ovid's Metamorphoses, and myths from Old Irish and Old Russian traditions.
Tuisto is a primordial god recorded by the Roman historian Tacitus in his 1st century CE ethnography Germania as the earth-born progenitor of the Germanic peoples. Some scholars link the name to a Proto-Germanic word meaning "two-fold" or hermaphroditic, which connects Tuisto to Ymir on both linguistic and mythological grounds, since Ymir also reproduces asexually from his own body.