Before deciding whether to purchase a plant, and how best to plan where to place it, the basic knowledge of how long it lives and how often it blooms is a necessity. What's the difference between annuals, biennials and perennials?
Annual means that the plant has a full life cycle (seed-to-seed cycle) in at most one year.
It will germinate, bloom and die that year. This is not a calendar year per se. Some species germinate in autumn, survive through the winter and bloom next spring.
A good example is the French Marigold.
A biennial plant takes two years to complete it's life cycle.
It will germinate and grow, survive through one winter, and in the second year it will grow more, bloom, and die. Biennials are less common than both annuals and perennials.
A well-known example is parsley.
Perennials are plants that live more than two years.
By this definition, many plants including trees are perennial, although the term is more commonly used for herbaceous plants.
A good example is a shasta daisy.
Lots of vegetables are grown as annuals, while they are actually biennials or perennials. For example parsley will be harvested in its first year, as are carrots. Onions are biennials that can be grown from seed in the first year. The resulting small onions can be planted again (you can buy these as well), to be grown to a full size onion. Peppers, tomatoes, etc, are tender perennials that are usually treated as annuals in the vegetable garden.
Because perennials generally last the longest, people often choose them first when designing their garden. Among other things, this broad category includes popular bulbs like tulips and daffodils, and flowers with many varieties, like lillies.
Learn2Grow offers detailed information about annuals, biennials and perennials, as well as links to other sites. This is an interesting quote from them about perennials.
The general definition of a perennial is a plant that lives for three or more years. The problem with this limited description is that some plants, like peonies, can live for over 100 years, while others, like some Agastache species, tend to only live for about four. Moreover, some perennials, like Monarda, bloom in the first year from seed, while others, like Baptisia, can take four years before they bloom from seed at all.