Article 100 defines a MWBC as:
"A branch circuit that consists
of two or more ungrounded conductors that have a voltage
between them, and a grounded conductor that has equal
voltage between it and each ungrounded conductor of the
circuit and that is connected to the neutral or grounded
conductor of the system."
Basically, it means the circuit has two or more hot legs and a neutral. Think of a 208 volt 3-phase Y, or the 240V single phase that goes to the range or dryer at your house. It could also be a shared neutral situation, for example, you have a duplex receptacle with the tab removed, a hot going to each recept, but only one neutral going back. Saves on wiring.
A yoke is just a mounting device, like the mounting hardware on a duplex receptacle.
Hope that helps.