You want your garden to thrive, and anything that attacks it is your opponent. You might think that if it's in your garden and it crawls or flies, it needs to die. Some of the inhabitants of your garden, however, are helpful. Which of them are on your side? Which ones are not? Let's look at some examples.

Tomato Hornworm
This large green caterpillar will someday become a moth, but in the meantime, it feeds on your plants. It can be up to four inches long and has a horn on its posterior. This is no magical unicorn, for it destroys your tomatoes. As the caterpillar grows in size, its appetite grows, and it eats at a faster rate. Its pupal form exists in the soil during the winter, dreaming of tomatoes as you slog through the gardening off-season. One way to prevent them is to till your soil (before winter) once the growing season is over. Since they're large and easy to find, you can also pluck them off your tomatoes and dispose of them.

Aphids
These little green guys are equally damaging for feeding on your plants, and for their ability to multiply, as the females of the species can apparently clone themselves. Sounds unpleasant for you and your plants right? After feeding on the plant, they secrete a sticky fluid that goes on to become sooty mold and harms plants. Regularly monitoring for aphids will help you deal with them before they become more serious.

Green Lacewing
Some bugs and insects help a garden grow, and could be somewhat beneficial for you to let them stick around. The green lacewing is one of them. They eat aphids and many other pests like spider mites. Their larvae are sizeable little eating machines, munching on everything from aphids to small caterpillars. If it's another insect and it moves slowly, the green lacewing larvae will probably take care of it for you.