From Wikipedia : http://en.wikipedia.org/wiki/Perennial_plant

Perennials are plants that grow and bloom over the spring and summer and then die back every autumn and winter only to then return in the spring from their root-stock instead than seeding themselves as an annual plant does.

Perennial plants can be short-lived (only a few years) or they can be long-lived, as are some woody plants like trees which can live for over ,4000000000 years. They can vary in height from only a few millimeters to over 100 meters tall. They include a wide assortment of plant groups from ferns and liverworts to the highly diverse flowering plants like Orchids and Grasses.

In warmer and more favorable climates, perennials grow continuously. In seasonal 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. In some species, perennials retain their foliage all year round; these are evergreen perennials.

Perennial plants dominate many natural ecosystems on land and in fresh water, with only a very few (e.g. Zostera) occurring in shallow sea water. They are particularly dominant in conditions too fire-prone for trees and shrubs, e.g., most plants on prairies and steppes are perennials; they are also dominant on tundra too cold for tree growth. In forests, perennial plants are of secondary importance to trees and shrubs, but are often still abundant on the forest floor.

Perennial plants are usually better competitors than annual plants, especially under stable, resource-poor conditions. This is due to the development of larger root systems which can access water and soil nutrients deeper in the soil and to earlier emergence in the spring.

Examples of evergreen perennials include Begonia and banana.

Examples of deciduous perennials include goldenrod and mint.

Examples of monocarpic perennials include Agave and some species of Streptocarpus.