Annual plants complete their lifecycle in a single season. You usually plant seed in spring or early summer (or purchase plants started in a greenhouse). The plants grow and flower through summer and die in the fall. Unlike perennials, this year's plants will not regrow from overwintered roots next spring, though sometimes seeds produced by annuals will sprout and grow the following year ("self-seeding").
Annual plants have many advantages:
Many flower from early in the season until they die in the fall, compared with perennial plants which have a comparatively short bloom time.
Whether you grow purchased plants or start from seed, annuals are relatively inexpensive to grow. Many are easy to grow if you provide the right site and soil preparation.
They are temporary. You can experiment with a wide range of colors, textures, and forms. If you don't like the results, you can do things differently next year.
Annuals are great for filling in bare spaces in perennial beds. When spring bulbs die back, for example, you can fill the void with annuals. Annuals also provide season-long interest in pots and containers.
Annuals are versatile. They range in size from bedding plants less than a foot tall to giants that grow 8 feet or more. Annual vines can climb 10 feet or more on trellises or other structures or they can ramble that distance along the ground. Many annuals perform best in direct sun and warm weather. Others prefer shade and/or cool. Some tolerate light frosts while others die at the mere hint of a freeze. Blossoms run the entire spectrum of the rainbow. Some annuals are grown for their interesting foliage colors or textures.
There are hundreds of species of annuals to choose from. Within many of these, there are also different varieties or cultivars (short for cultivated varieties) to choose from. Varieties within a species may differ in flower color, size, disease resistance, or other characteristics.
A series is a group of closely related varieties that usually differ only in flower color. Some varieties are F1 hybrids -- grown from seed from a cross between two specific parents of the same species. Hybrids are often more vigorous than non-hybrid varieties or have special characteristics. You cannot expect that plants grown from seed produced by hybrid plants to "breed true." The next generation of plants will likely be substantially different from their F1 hybrid parents.