We have three types of weasels in Pennsylvania:
Long-Tailed Weasel - Mustela Frenata
Least Weasel - Mustela Nivalis
Short-tailed Weasel (Ermine or Stoat) - Mustela Erminea
The links above have photos and maps of the counties in which each species has been recorded. The Pennsylvania Game Commission also has a nice wildlife guide about PA's weasels.
Weasels belong to the family Mustelidae. This family includes otters, badgers, minks, and martens. Weasels are the smallest members of the family. Being carnivores, they eat meat; but as weasels are only around 5-10 inches long, they eat tiny creatures like mice and voles. They are quick and alert. They have a keen sense of smell. They're generally quiet and solitary, but they do make trilling sounds when they are happy and squeaking sounds when they're upset. In northern regions, weasels may change color from brown to white in the winter.
Even though I live near weasels, I can't say that I've ever seen one. But I found some videos of them online. In the first one, you get a really nice look at a long-tailed weasel. In the second one, you can hear the sounds that the weasel makes - first a chirping and then its trilling sound.