Hubs (also called market centers) are common places for market participants to trade gas or electricity. Hubs are used for physical transactions, as pricing points for calculation of price indices, and in some cases as locations for futures trading. Physical hubs are locations where multiple gas pipelines and or electric transmission lines come together, making it easy for trading to occur. But hubs may also be virtual, meaning that physical deliveries actually occur at other nearby points. Hubs commonly used for trading are sometimes also called market centers.
This graphic shows the various pipelines that interconnect at the natural gas Henry Hub in Louisiana:
And this graphic shows common electricity trading points in the U.S.
And these are North American trading points for natural gas.
And finally, key gas trading hubs in Europe plus the JKM, which is a virtual hub used to index Asian LNG prices.