They're Orb weavers:
Orb weavers (Araneidae) are often brightly coloured with rounded abdomens, some with peculiarly angled humps or spines. However, there is considerable variation in size, colour and shape in this group. They are often recognized for building beautiful, large, round webs, on which they rest, head downward, waiting for prey. The webs consist of a number of radiating threads crossed by two spirals. The inner spiral begins in the centre, winds outward, and is made of smooth threads like the radiating threads. It covers only the central 1/3 of the web. The outer spiral begins at the edges and winds inward. It is made of more elastic, sticky threads, coated with a liquid substance. One of the largest and most commonly encountered members of this group is Argiope aurantia, the yellow garden spider.More on the web (pun unavoidable) and their prey:
Like all spiders, black-and-yellow argiopes are carnivorous. They spin an orb web to capture small flying insects such as aphids, flies, grasshoppers, and Hymenoptera (wasps and bees). A female can take prey up to 47mm in diameter, up to 200% of her own size (Nyffeler et al. 1987)[Editor: that's almost 2" in diameter!! Think small bird or bat.]
The web can be up to two feet across. The spider hangs, head down, in the center of their web while waiting for prey. Often, she holds her legs together in pairs so that it looks as if there are only four of them. Sometimes the spider may hide in a nearby leaf or grass stem, connected to the center of the web by a nonsticky thread which quivers when prey lands in the web.
...The entire web is usually eaten and then rebuilt each night, often in the same place.
As for why we see them in the fall?
Yellow garden spiders breed once a year. The males roam in search of a female, building a small web near or actually in the female's web, then court the females by plucking strands on her web. Often, when the male approaches the female, he has a safety drop line ready, in case she attacks him. After mating, the male dies, and is sometimes then eaten by the female.And while all spiders are poisonous "these spiders are not dangerous to people, and their bites result in nothing more than a sore, itchy swelling that goes away in a few days."
...She guards the eggs against predation as long as she is able. However, as the weather cools, she becomes more frail, and dies around the time of the first hard frost.
In the spring, the young spiders exit the sac and are so tiny that their collection of bodies look like dust gathered inside the silk mesh. Some of the spiderlings remain nearby, but others exude a strand of silk that gets caught by the breeze, carrying the spiderling to a more distant area.
[Wikipedia: Argiope aurantia]