Basically it was just a fruit tree. It had no magical powers as far as we know. God simply told Adam and Eve that this tree was off limits so that they could choose to obey or not, however they had no idea what evil was before they first disobeyed God so eating the fruit and disobeying God allowed them to know what it was like to do something wrong, thus the name "tree of knowledge of good and evil" hope I explained that clearly :)
The tree of knowledge is in the garden of Eden, which is were Adam and Eve lived until they were cast out for eating a fruit from the tree of Knowledge. Usually it is portrayed as an apple tree but it was never specified in the Bible.
The tree of knowledge was a tree which, if you ate it's fruit you gained the ability to differentiate between good acts and bad acts. As far from what i understand, i believe that the only sin god would blame man for was disobedience, as they did not know what was good and what was bad, and hence he could not blame them for committing good or bad, the same as we could not blame a man for killing another man, if he did not know that killing another man was bad. but once Adam and Eve ate the apple they gained "knowledge of good and evil" and hence were liable for their sins. Just my opinion. Please no religious arguments.
Skeptics often complain that God set Adam and Eve up to fail. However, God had to give Adam and Eve a choice. Without free will to choose, Adam and Eve would have been mere puppets. True love always requires choice. God wanted Adam and Eve to choose to love and trust Him. The only way to give this choice would have been to command something that was not allowed. Since God had planted in the garden all the different trees from which we now get fruit,1 the test was not too difficult. Adam and Eve had plenty to eat and a large variety of fruits from which to choose, and could have chosen to believe God. They were only commanded not to eat from one tree out of the many.
It was one of two trees that stood as symbols of Gods only law in those earliest times. That particular tree was a fruit tree of some sort. It stood in the garden of Eden. It wasnt the tree itself or even the fruit that was 'magic'. The 'knowlege' of sin and wrong doing came with disobience to God and choosing satan over God allowing evil to destroy perfection.
Honestly I don't know if there was two different trees in the garden of Eden. There may have been only one that they could eat of freely until its fruit became an illustrious thing through the temptations the serpent tempted eve with.