From Envirowiki

A pesticide is a chemical or process designed to destroy pests. Pesticides are frequently used on agricultural crops, to destroy insects and other organisms, that usually become pests through the monoculture effect - they grow in numbers due to the abundance of food, and lack of predators.

Modern pesticides are often fairly toxic, being petroleum based, and can pollute waterways and surrounding ecosystems, killing off insects and other organisms that are a vital part of the food webs.

[edit] non-toxic pesticides

There are numerous non-toxic pesticides that work against some pest without destroying others, and are relatively safe for the surrounding environment. For example, a plate full of beer will attract and drown slugs and snails, which often are a problem in the garden. But often the most productive way of getting rid of pests is by attracting predators - predatory insects like praying mantises and ladybugs eat other insects, while chickens love eating snails, and provide eggs with the protein the get from eating them.