A Quest Hub is a location in a zone that contains a number of NPCs that gives quests. Usually just one NPC per location gives quests that are part of the zone progression, and the others are just for extra EXP and fun. When you finish the last progression quest at a hub, you are usually directed to the next hub in the series.
There is a variation on this, common at the faction camps in Fens of Nathsar (in EverQuest II), for one, where the camp has a large number of NPCs, most of whom give only one quest. Often the NPC you were directed to will withhold his quest until you meet some criteria, such as completing all the other quests or reaching a certain faction threshold, then he will offer a quest that directs you further into the zone to another camp. This arrangement is referred to loosely as a Quest Wheel, but the location is still denoted as a Quest Hub.