They are quite familiar with the pests in your area and methods of control. There are numerous products available in all kinds of packaging. For small gardens, hand spray bottles or aerosol cans are effective and easy to use with little danger of exposure. These range from the least dangerous products such as Safer , to the mixed insecticide and fungicides in aerosol cans. Each has their advantages and disadvantages.
Pesticides also come in dusts, granular, and wettable powders as well as liquids.
For large gardens, buying pesticides in the concentrated form and mixing yourself is more economical, and potentially dangerous. When you are at the store to purchase a pesticide, read the label and see what pests they control. Also, look at the warning or rating label, see what the active ingredient is, whether it is toxic or not to plants, animals, children, or the environment, and if it meets your personal preference. Don't be afraid to ask questions. A broad spectrum product controls a wide range of pests, while selective products are very narrow in control.