You have a Writing Spider or Argiope aurantia. You can find additional information here: http://davesgarden.com/guides/bf/go/69/

THAT IS YOUR COMMON YELLOW GARDEN SPIDER WE HAVE THEM ABUNDANT AROUND WHERE I LIVE IN NORTHEAST WISCONSIN AND YOU SHOULD BE WARY AND CAREFUL AROUND THESE AS MY DAUGHTER WAS BIT A FEW YEARS BACK BY ONE WHILE RUNNING THROUGH THE FIELD BEHIND OUR HOME AND IT GOT VERY INFECTED THEY ARE SOMEWHAT VENOMOUS, THEY ALSO LIKE MANY OF THE SPIDER FAMILY WILL REPRODUCE IN NUMBERS AND WILL BE AROND YOUR AREA IN NUMBERS BY THE END OF SUMMER THEY LIKE TO SPIN WEBS ACROSS TRAILS AND PATHWAYS ECT. AS THIS IS AN IDEAL OPEN AREA TO CATCH THIER PREY OR... CHILDREN AS IN OUR CASE! LOL HOPE THIS HELPS,BOB