Epic Fantasy describes books involving vast expanses of land and civilisations and with stakes that affect the entire world. They often have a large cast of characters, require maps to clarify the geography, and are usually much longer in length. High fantasy is similar (apparently) but tends to focus more on the characters than the world they live in (even if ther story affects the whole world).
Epic fantasy involves a quest to rid or conquer the world of dark magic, an evil ruler, or a magical artefact that could alter the world as they know it. (without a quest or journey, the impact on the entire world can not really be explored, so in that respect, the journey or quest is a crucial element in this genre). High fantasy also features a quest or journey, but the characters’ actions and decisions take precedence over the effect on the world as a whole.
** This isn’t a particularly long post as I need to do some more reading on this topic.