The Tree of the knowledge of good and evil, is one of the two trees situated in The Garden of Eden, the other being Tree of Life. God had commanded Adam the first man, and his wife Eve to not eat the fruit of the tree of knowledge of good and evil. Satan, unfortunately, in the form of the serpent tempted Eve and Adam to eat the fruit causing the fall of man.
The exact appearance of the tree itself and the fruit it produced is not known. It must have looked very appealing, however for the text of Genesis says the tree appeared good for eating and to make one wise.