Violets are divided into two main types.  If you look at the leaves and flowers; their petioles (the flower and leaf stem) either branch from an aerial stem or appear to grow directly from the ground.    If they appear to sprout from a stem, they are considered stemmed varieties.  If they appear to sprout from the ground, they are considered stemless.  Sometimes early in the season, stemmed varieties may appear to be stemless, but if you look closely, you will be able to tell the difference.