Think of your garden as a living system. Plants and animals create a web of interactions. This web is what we refer to as "habitat." Habitats do not obey arbitrary lines like county lines, fences, property lines, etc. They are overlapping systems with subsystems of their own called niches. For example a stream in the woods is a riparian niche within a general woodland habitat.
To start identifying your habitat, begin by examining basics like how much sun and water does your property receive? What is your soil type? Do you have rich black humus, or hard clay? What kind of moisture do you have in areas of your garden? Are some areas dry and some wet? Next, get to know the habitats and plant communities that would be appropriate in your neighborhood. Look at nearby undisturbed sites to learn what habitat is most natural in your area.
The importance of planting with native species and in accordance with your habitat zone cannot be understated. Native ecosystems are vanishing rapidly due to suburban development, logging, mining, etc. Some exotic ornamentals (plant species that have been introduced from another part of the world) that have been bred for hardiness end up in the wilderness choking out native species. Blackberry, morning glory, and ivy are familiar examples of this. Much water, effort, chemical pesticides and herbicides are wasted on planting other, less hardy, exotic ornamentals that are not truly suitable for one's particular habitat. And who knows? In time, you may see the return of some of the birds, butterflies and animals that once lived on the ground you own.