The Tree of Knowledge of Good and Evil appears in the Book of Genesis as a key element in the fall of man from God's grace. When God created Adam in the Garden of Eden he gave him permission to eat from any tree except that of the Tree of Knowledge.
Genesis2:17 But of the tree of knowledge of good and evil, thou shalt not eat. For in what day soever thou shalt eat of it, thou shalt die the death.
But Adam did eat the fruit of the tree after being induced to do so by Eve, and both of them were cast out of paradise.
The Bible does not really say what sort of tree it was, but it is historically portrayed as an apple tree; more likely, it was a fig tree.