A cartel refers to a large group of goblin traders. Goblin cartels often compete with other cartels, such as in the Trade Wars. Cartels are normally led by trade princes, but they don't have to be; although Venture Company is a cartel, its leader is not.
After the goblins overthrew the Zandalari trolls, they celebrated their new found freedom by turning on each other, creating chaos as countless feuds were waged and alliances formed. Soon enough they were split into various different groups, with the most powerful of these groups known as the cartels. However, as the battles raged on and it became clear no one was actually winning, the cartels realized it was in their mutual interests to broker a truce. Despite this, the conflict would never really end, as the goblins now turned to trade, doing battle in the economic arena. For years to come the cartels were rivals, constantly outshining and outsmarting each other as they continued in their quest for wealth and power.
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 (later shown to be false) and are the largest grouping of goblins within the Trade Coalition. The word was used by Chris Metzen as a synonym for a goblin faction and/or the private army of a Trade Prince. 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.