Copperhead Photo by John White
Timber Rattlesnake Photo by John White
Cottonmouth Photo by John White
The copperhead is the only poisonous snake found across all of Maryland and Virginia. Though its bite is almost never fatal to humans, it can be (rarely) fatal to a terrier who is likely to be bit several times if it finds one in a hedgerow or prowling for mice near a barn.
The good news is that copperheads are not quite as common as some people believe -- most snakes believed to be copperheads are misidentified. Nonvenomous snakes that look a bit like copper heads include eastern milk snake and the northern water snake, but these snakes have round pupils while a copperhead and other poisonous snakes have yes with vertically slit, elliptically-shaped pupils.
Cottonmouths are rarely found far from water and generally only in very swampy parts of the most extreme southern parts of Virginia. Just because you see a large black snake swimming does not mean it is a cottonmouth -- it it much more likely to be a black rat snake.
The timber rattlesnake, or canebreak rattlesnake has a wide variety of color forms (see pictures), but is generally found only in the western parts of Virginia and Maryland, generally in the mountains near rocks and ledges. As the "canebreak" moniker suggests, however, they are very infrequently found on the coastal plains.