A cartel in Warcraft normally refers to a large group of goblin traders. Goblin cartels often compete with other cartels, such as in the Trade Wars.
The now non-canon RPG, while having quite a bit of lore on goblin organizations, largely ignored the known cartels and never defined the term. Cataclysm and post-Cataclysm materials seem to imply that Cartels are each run by a Trade Prince and are the largest grouping of goblins within the Trade Coalition. It is unknown what role Moguls play in Cartels (though they seem to have some clout), as Gallywix was not said to be one even as an entrepreneur seeking to overthrow the Trade Prince that he was under.