One area of particular concern is the toxicity of herbicide use to non-target species such as bees, birds, and people. Federal law requires testing of pesticides before registration, and California is unique because the state requires additional testing and regulations regarding the use of pesticides. Knowing the specifics of the tests and regulations regarding herbicide use can be beneficial when deciding if and when the use of a product is required.
One place to start is understanding the general definition of toxicity: UC IPM Toxicity
The National Pesticide Information Center has a lot of information about how pesticides interact with soil, water, and other aspects of the environment. Their fact sheets for individual pesticides are a good way to get more information about specific products.