Once upon a time, this referred to artificial flowers that were hand-made out of silk, sometimes hand-painted. Silk flowers were popular for hats, dresses, and wedding bouquets, because of their durability. However, today "silk" flowers are usually nylon or some other synthetic, making them much cheaper but without that lovely texture. Artificial flowers are still used for all of the above, plus decorating, grave memorials, and so on. Some are pretty realistic, and a recent trend has been to add artificial dewdrops, thus increasing the trump l'oeil effect. You can buy silk flowers everywhere from Wal-Mart to specialty hobby stores.