This is a Cross Orbweaver spider (Araneus diadematus). These spiders are very common throughout North America and are named for the distinctive markings on their abdomen. Most of them seen in webs are females, since the males tend to wander looking for a mate and rarely spin their own web.
These spiders are not aggressive and will flee the web if threatened. They are not considered dangerous to people or pets. In fact, they are considered to be beneficial to humans because they eat a lot of pest insects.
Toward the end of autumn, the females will lay their last clutch of eggs, and then die at the first frost. The eggs survive the winter, even withstanding freezing, and the warmth of spring causes the eggs to hatch, releasing a new generation of orb weavers into the world.