The Acheiropoieta (αχειροποιητα, "Made Without Hands") are a collective name for a variety of icons which are believed to have been created, as the name directly states, without human hands. There are many of them (as far as I know, all of either Jesus Christ or the Theotokos), and their connection is mostly in their origin, so instead of trying to describe them in general, I will shortly describe most of the most famous ones separately.