A marketplace where brokers post freight and carriers look for it.
Subscription services where available loads are listed with origin, destination, equipment and often a rate. The main ones also carry broker credit information and rate history.
Load boards show you what is available, not what is profitable. The same load is visible to every other truck in the market at the same moment.
Related
Inteleri works out what a load actually pays you once things like this land. See how it works.