Rather difficult to define. From scientific, botanical point of view these are cacti that live on other plant (trees) rather than on earth itself. In common usage this name more often than not denotes so-called leaf cacti that have stems flat enough to resemble leaves and no spines whatsoever. They come from tropical areas, rainforests etc., and that means they need water and shade and warmth (but still some porous potting mix is OK), ie. not below 10 centigrade in winter. In European climate they can spend summer out, eg. hanging on a tree branch in a hanging pot. Here we include Schlumbergera (Christmas cactus), Epiphyllum (Easter cactus) and also Rhipsalis, Lepismium, Disocactus etc. Recent fashion amongst botanists make them change the genus names every second month so it is better not to notice and let the children play their game (otherwise they may start crying and this is unpleasant to listen to).