This is growing in the woods near my house in the Butler, Pennsylvania area. I have not seen it around before and I have no idea what is it. What is it? Is it poisonous to pets (or little kids)?
That looks like a bit like a Smilax to me, possibly Smilax herbacea.
Make sure you have a positive ID on any wild plant before you eat it, but if this is Smilax it looks like you don't have to worry about it being poisonous: from Flora of North America entry on the genus Smilax:
For you, or for other Pennsylvania people, note that the Pennsylvania Flora Project of the Morris Aboretum is a great resource. You can download species lists by county, and just that and some google image searches can often substitute for a field-guide of your county.
If you want to identify plants the hard way, and are willing to deal with special botanical vocabulary, the Flora of North America that I linked to above is the definitive resource.