A photo would certainly help, but usually these colorful spiders like this, especially in large webs, are garden orb weavers, One very colorful one is the orchard spider, a type of long jawed orb weaver.
Common yellow and black spiders are the Argiope garden spiders.
All of the orb weavers are very harmless, and this is one of the largest family of spider in north America with over 160 species.
These spiders are very beneficial in plants, gardens, and shrub.
Here are a couple likely examples:
http://bugguide.net/node/view/2010/bgima...
http://bugguide.net/node/view/2025/bgima...
There are a number of other possibilities, but you need not worry about such spiders, I welcome them.
By "this" spider, do you mean the one in your photo?
I found this spider in my backyard. It is yellow with a green(?) belly. Anyone know? It's about 2 inches in diameter. And if it helps, I live in California. Thanks!
