Strictly speaking, an annual plant is one that completes its growing cycle (grows from seed, flowers and produces seed) in the course of a single growing season. In other words, annuals pack a lot of living into a short span of time.
But beyond this simple definition, there is an even wider range of plants that we treat as annuals. Some, such as impatiens, heliotrope and tuberous begonias, are actually tender or "half-hardy" perennials that can't survive even a light frost. On the other hand, some annuals, such as pansies and ornamental cabbage, are extremely cold-hardy and can withstand freezing temperatures quite well.
One of the best things about annuals is their incredible diversity and versatility. Using them allows you to compose really exciting combinations of color, form and texture that will last all season long. Colors range from bright midsummer favorites such as zinnias and Mexican sunflowers, to the subtler pastel shades of stock or lavatera.
You can also select annuals for your garden based on characteristics other than flower color. There are annuals that are tall, medium, short or climbing; ones that prefer either full sun or partial shade; and those with special virtues, such as delightful fragrance (stock, mignonette, nicotiana) or attractive foliage (caladium, coleus, dusty miller).