They are really just smaller versions of the larger perennial dahlias that are grown for their big bold flowers and flashy colors. Technically, annual dahlias produce smaller versions of the same perennial roots that can survive through winter to regenerate the following spring. Yet to most of us, it is easier to purchase new plants in spring than to grow new plants from stored roots.
Relative to larger dahlias, everything about annual dahlias is subdued. The flowers are neither as big, nor as variable. They can bloom any color except blue or green, but are usually simple shades of red, orange, yellow, pink or white. Their main advantage is that they do not get much more than a foot tall, so they fit into more situations and do not need to be staked.
Bloom starts rather late in spring and continues until about now. Some varieties have bronzed foliage. Fertilizer promotes bloom and healthy foliar color; but too much nitrogen can inhibit bloom. Dahlias can be happy in pots, but only with good drainage and regular watering.