Excerpt from: Sustainable Urban Landscape Information Series (SULIS), University of Minnesota.
By definition, annuals are plants that complete their lifecycle in a year. They add masses of color to a landscape in the form of blossoms and sometimes foliage as in the case of Coleus (Solenostemon scutellariodes). Annuals can be planted in containers, hanging baskets, on trellises or directly in a garden bed, and many, such as pot marigold (Calendula), bachelor's buttons (Centaurea cyanus), and poppies (Papaver species), will self-sow for more plants next year. Great for borders, containers or mixed in with leafy perennials such as hostas (Hosta species) or lamb's ears (Stachys byzantina), annuals are easy to grow and maintain, and they bring color and texture to a landscape throughout the season.
Biennials require a little more patience than annuals or perennials, but the results are worth the wait. Biennials usually require two growing seasons in order to complete their lifecycle. Be aware that biennials produce foliage and roots the first year, but do not bloom or produce fruit until the second year - a point to be emphasized when planning a landscape. After the second year, some biennials will die while others will self-sow, creating new plants for the following season. Some popular biennials are foxglove (Digitalis), hollyhocks (Alcea), sweet William (Dianthus barbatus), and Canterbury bells (Campanula medium).
Perennials are usually long-lived plants that have the distinct honor of being the backbone of any garden. Many are relatively hardy and require only minimal care such as minor pruning and watering throughout the growing season. Perennials can be easily propagated by division, allowing gardeners the opportunity to change their garden design every few years with little investment beyond time and effort.