The difference between annual and perennial plants is simple: annuals complete their life cycle within a year, while perennials live for over two years. A third classification, biennial plants, refers to plants with a two-year life cycle. Annuals may have a life cycle of any duration under a year; some have life cycles of only a few weeks. Perennials may live for just a few years or for well over 20 depending upon the species of plant.
Annual plants that naturally complete their life cycle in under a year are known as true annuals, but some biennials and perennials may be grown as annuals in certain contexts. For example, some annual plants may be perennial in their native habitat, but are not hardy enough to survive winter in the environment in which they are grown. Certain root vegetables, such as carrots, are biennials that are treated as annuals, harvested in their first year for the strong root that grows to provide nourishment to the plant in the second year.
Annual plants fall into two major groups: summer annuals and winter annuals. Summer annuals go from germination to death within a single season, be it summer, spring, or fall. Many summer weeds are of this variety.
Winter annuals are longer lived. They germinate in the fall or winter, blooming later in the season or even as late as early spring. Winter annuals fill an important niche in many ecological systems, as they provide ground cover when perennials are dormant.
The term perennial is often understood to refer to perennial herbaceous plants, as all woody plants are perennial by definition. Perennial plants are very diverse. Some, called monocarpic plants, bloom and fruit only once, while most, called polycarpic, do so every year. Perennials, as you may imagine, are hardier than annual plants and have evolved structures that enable them to survive for many years, such as bulbs and rhizomes. Perennials may be deciduous, alternating periods of growth and dormancy in response to climate changes, or evergreen, growing year round.