Short answers, pulled from the story.
Lloyd Alexander coined the term "high fantasy" in a 1971 essay titled "High Fantasy and Heroic Romance," first delivered as a talk at the New England Round Table of Children's Librarians in October 1969. Alexander drew on Northrop Frye's Anatomy of Criticism, specifically Frye's Theory of Modes, to define the concept.
High fantasy is usually set in an alternative, fictional secondary world with its own internal rules, while low fantasy is characterized by being set on Earth or a familiar, rational fictional world with the inclusion of magical elements.
The romances of William Morris, such as The Well at the World's End, are sometimes regarded as the first examples of high fantasy. James Branch Cabell's Biography of the Life of Manuel series, published starting in 1919, and E.R. Eddison's The Worm Ouroboros (1922) and Zimiamvian Trilogy (1935-1958) are also early works in the genre.
J.R.R. Tolkien's works, especially The Lord of the Rings (1954-55), are regarded as archetypal works of high fantasy. They set the benchmark against which other works in the genre are measured.
The conflict between good and evil is a central and defining theme in high fantasy, and the importance placed on this moral struggle is widely regarded as what distinguishes high fantasy from sword and sorcery. Some works treat the conflict as a profound ethical concern; others frame it as a power struggle.
Dungeons and Dragons is a prominent role-playing game associated with high fantasy. Its campaign settings include Dragonlance, created by Tracy Hickman and Margaret Weis, and Forgotten Realms, created by Ed Greenwood, both of which have served as a basis for many fantasy novels.