Pictured above is the large banded garden spider, Argiope trifasciata. This type of spider is an ORB weaver, spinning their webs into circles commonly seen in gardens.
Below is pictured the hunting white-banded crab spider, Misumenoides formosipes. A predator with no web spinning abilities it attacks its prey, hopefully a garden pest!
While they both look kind of strange and creepy neither one of them is dangerous; to humans that is...