Size and arrangement. Look at Switchboards and Panelboards in a manufacturer's catalog. Individual sections of switchboards are sometime referred to as panels, but probably not correctly.
One of the old original differences was that a switchboard was the term used when the back was accessible, whereas it is a panel when it is mounted against the wall. This general difference is probably not accurate anymore, though.
Another way to look at it is a switchboard is floor-mounted, while a panelboard is wall-mounted (although very large ones might reach the floor). One other criteria that we use is up to 1200 amps - panelboard - 1600 amp and above is a switchboard.
The terms switchboard and distribution panel are basically interchangeable today. At one point we used "switchboard" when it contained a series of fusible switches feeding loads and "distribution panel" when it contained circuit breakers. "Switchgear" is a general term which includes both of these as well as say, panelboards, motor control centers, unitized substations, etc....
Jamie,
As for switchgear, take a look a jraef's link. Switchgear has a very specific UL and ANSI definition and should not be confused with switchboards or panelboards.
Switchboards have different construction requirements that a distribution panelboard....Any panelboard can be called a "distribution panelboard if it has less than 10% of its OCP's serving 120 volt loads. In addition a switchboard can have both vertical and horizontal bussing. vertical bussing. the switchboard bussing ampacity can be tapered based on its designed load and other criteria
