The circular object with concentric circles of blue and white is known as the evil eye. If you look closely in this photo, you will also noticed bracelets with much smaller versions of the same symbol. This photo was taken in Istanbul, Turkey, but I have seen this bracelet in Greece and other places in Eastern Europe.
The belief is that is that if you wear one of these amulets, bad things won’t happen to you. I’m not sure if there is any science to back this up. The vendor’s missing foot and toes make me wonder about its efficacy.
Incidentally, the sign that we sometimes use to mean “OK” in western culture is the “evil eye” sign in eastern Europe. So don’t flash the “OK” sign in Istanbul – it may cause a fight.