What is a Burned County?

What is a "burned county"exactly? I use the term in the heading of this site but it has come to my attention that I have never bothered to really explain it. A "Burned County" is simply a rather more dramatic name for what the Library of Virginia calls a “lost records locality.” 

And what does the Library of Virginia say of these?

Numerous Virginia localities, most of them in the eastern part of the state, have suffered tremendous losses of their early records because of intense military activity (predominantly during the Civil War), courthouse fires, and/or natural disasters. At some point, almost everyone conducting genealogical or historical research will face the problem of finding information from a county or city described as a “lost records locality.” 

The Library of Virginia lists some 22 Virginia counties as having what they classify as "catastrophic" levels of loss. And 23 more at "considerable level" of loss.

The Civil War accounts for the greatest number of these catastrophic losses but not all. King William County's  loss came in the winter of 1885 originating probably from an untended stove. Appomattox lost its courthouse to  a devastating chimney fire in 1892. Buchanan County's records were destroyed by fire 1885 only to have later records damaged by flood in 1977. New Kent of course suffered the dual losses from the Great Fire in Richmond during the Confederate evacuation of 1865 and the early loss from the hand  of John Posey during his arson in the summer of 1787.