This is poison ivy! Sometimes the leaves have notches, sometimes it has a shiny reddish appearance. The constant characteristic is that the leaves grow in threes. Getting the oil on your skin can cause a miserable itchy rash. Most of you know this, but I want to warn those who don't!
"Leaves of three, let it be!"
And here's a photo of poison oak, below. Terri left a comment on this post about poison oak, so I added this photo. It looks like a baby oak tree. It would be tempting to pick some of this for a floral arrangement. Note to self: Don't do that!