A perennial (Latin per, "through", annus, "year") is a plant that lives for three years or more- in most cases a lot longer than three years. Herbaceous perennials are plants that do not form woody tissue and the term perennial more commonly describes these plants, since woody plants (i.e., trees and shrubs) are always perennials.
In warmer climates, perennials grow continuously. In colder climates, their growth is limited to the growing season. For example, in temperate regions a perennial plant may grow and bloom during the warm part of the year, with the foliage dying back in the winter. These plants are deciduous perennials. Regrowth is from existing stem tissue. In many parts of the world, seasonality is expressed as wet and dry periods rather than warm and cold periods. Some perennials retain their foliage all year round; these are evergreen perennials.
Examples of evergreen perennials include Agapanthus (Lily of the Nile), Penstemon, Coreopsis and Heuchera (Coral Bells).
Examples of deciduous perennials include Alstromeria (Peruvian Lily), Dahlia and many blubs.
Perennials are benificial to the beauty of your garden, as most will provide both flowers and foliage all year. Annuals are short lived showy palnts, but need to be replaced each season. Perennials are also hartier than annuals because they develop larger root systems that help them overcome any harsh conditions they might face.
Perennials can be grouped by hardiness. For example, varieties that flourish in Missouri may not survive a Minnesota winter. In the United States, the Department of Agriculture publishes a hardiness map, using average minimum temperature ranges to form a zone numbering system. The higher the zone number, the less hardy the plant. For example, a zone 8 perennial, suited to a minimum temperature range of 10 to 20 degrees F, will not survive a normal zone 4 winter, which has a minimum temperature range of -30 to -20 degrees F.