Short answers, pulled from the story.
Lloyd Alexander coined the phrase high fantasy in October 1969 during a speech to the New England Round Table of Children's Librarians. His essay appeared later as High Fantasy and Heroic Romance in 1971.
High fantasy builds entirely separate universes with their own internal rules that differ fundamentally from the primary world we inhabit today. Low fantasy remains tied to Earth or rational settings where magic intrudes on the known world.
William Morris published The Well at the World's End in 1896 as an early example of the genre. James Branch Cabell began releasing his Biography of the Life of Manuel series starting in 1919.
J.R.R. Tolkien released The Lord of the Rings between 1954 and 1955 in three volumes. His work became the archetype for all subsequent high fantasy fiction.
Tracy Hickman and Margaret Weis created the Dragonlance campaign setting for role-playing games while Ed Greenwood developed Forgotten Realms as another popular universe for tabletop adventures. These settings draw directly from high fantasy literature to build their worlds.