This is an enormous genus of species and cultivars. The majority have flowers that have the classic daisy appearance with a central disk and rays of petals radiating like a child drawing of a sun.
The type that most people know are the Michaelmas daisies, but there are many other garden worthy plants that are well worth exploring, in particular the long-flowering Aster amellus and Aster frikartii varieties.
On the whole the asters are very easy to grow. The main problem is mildew, but fortunately this is rarely fatal and it is something that can be lived with, especially if the foliage is masked by plants in front.
Many asters, especially the Michaelmas daisies, look terrible in pots and therefore never look at their best in garden centres or nurseries. Take this into account when buying.
They are all shallow rooted and can easily be moved even when they are in full flower. This can be a useful technique for filling gaps in the border. Water a clump, dig it up and replant and water again; it will not notice the move. The Michaelmas daisy types should be divided every three or four years.