Dahlias are mid- to late season flowers that come in a vast array of colors and forms. There are small-flowered bedding dahlias as well as plate-sized blossoms on 6 ft. plants. It's hard to give an overview of dahlias when there is so much diversity.
Dahlias are often grown for their long-lasting cut flowers. There are some 30 species of dahlias and over 20,000 cultivars. Native to Central America, they have become darlings of plant breeders and flower shows and every year they become more dazzling. The forms and colors of the flowers vary with the classifications. They are often labeled as perennials, but they are actually too tender to leave out all winter, in all but the warmest climates. Which means you either have to dig and store the tubers or grow them as annuals and replace them each year.