Kind of old and I apologize for resurrecting a "zombie thread", but I don't think the entire question was answered and I happen to know this very well.
"Switchgear" (SWG) and "Switchboards" (SWB) (two different things) are for power distribution, meaning they take in primary or secondary voltages at a site and distribute it to other equipment. In general, SwitchGEAR is HV (above 25kV) and MV (1kV-25kV) and involves equipment that contains very high energy "cells" that, if something goes wrong, will isolate and contain the damage to that one cell so as to not shut down an entire facility or area. SwitchBOARDS are typically down stream of SWG and are the distribution point for OTHER loads, such as transformers, Panelboards (PBD) and Motor Control Centers (MCC). There are different ANSI and UL standards for SWG vs SWB, mainly because the intent is for SWG to "hold in" and take the abuse of a fault for a relatively long time while waiting for some other device further down stream (SWB, PBD or MCC) to act on it to clear the fault. So the SWG is the "last line of defense", but would have the widest reaching effects if it is the thing that must clear the fault.
Motor Control Centers are basically the same as SWB construction (they grew out of that industry in the 1950s), but are generally used to provide power to motors, not general loads. They are different from SWBs in that you have 'cells" (called "buckets") that contain the control equipment so that in theory, you can disconnect power from one bucket and allow the rest of the machinery powered by that MCC to continue to run. You cannot do that with a typical SWB, if you had to remove one breaker or switch, you have to kill power to the entire SWB. There are exceptions to this rule, but conceptually, this is what it boils down to.
A Power Distribution Center (PDC) is basically just the name for a factory-built structure, a portable building, designed to HOUSE equipment such as SWG, SWBs, Transformers, PBDs, MCCs and/or other controls or instrumentation, in any combination or configuration the end user desires. There is no standard definition of what goes INTO a PDC, but there are several standards as to how they are built, depending on where they are going and what they are housing. Some must be built to withstand explosions, such as in a refinery hazardous area, some must be able to withstand extremes of weather such as high winds, crushing snow, marine salt spray etc.