The life cycle of a plant can be looked at as having four stages. The plant begins as a seed; goes through the growth process of creating roots, stems, and leaves; flowers; and returns to seed. The classification system of annuals, biennials, and perennials informs you how a particular plant goes through this cycle. Keep in mind, however, that plants interact with their growing environment, and climate may lead them to behave differently than the classification would lead you to expect.
Annuals: Annuals are plants that go through a complete life cycle, from seed to seed, in one growing season. Annuals may reseed and grow in the next season, and their reappearance year after year can lead people to mistake annuals for perennials. Popular annuals include cosmos, impatiens, marigolds, petunias, snapdragons, and zinnias.
Biennials: Biennials take two years to go through a complete life cycle. The first season's growth only manifests a few, low-lying leaves, and the rest of the cycle takes place in the second season. Commonly used biennials include Canterbury bells, hollyhocks, and sweet William.
Perennials: Perennials continue growing, blooming, and seeding for a number of years. The root system or bulb, and sometimes the stem and foliage, last through the winter. Keep in mind that the garden's zone can affect how perennials behave — grown in a warmer locale, they may act like annuals. Favorite perennials include columbines, delphiniums, irises, lilies, peonies, and tulips.
Making choices: Knowing about a plant's life cycle can help you make good choices when planning a garden because you will know if a plant needs to be replaced every year, will only flower in the second year of growth, or will flower every year for more than two years. Other factors to consider in choosing plants include the soil quality, the degree of light/shade, and your hardiness zone, remembering that climate may induce a plant to act uncharacteristically. Resources like your state's Cooperative Extension Service (run through local colleges and universities) and the National Gardening Association can provide more detailed information for your particular area.