When God created man, He gave him two different faculties. One is nafs-e-ammarah and the other is nafs-e-lawwamah. (75:2, 12.53)

In other words, these are ego and conscience.  Nafs-e-lawwamah is the conscience. Nafs-e-ammarah is the ego. A human being has two faculties: ego and conscience. Ego is the entry point for Satan. Conscience is the entry point for angels. This is the situation of human beings. If a person comes under the influence of ego, he will allow Satan to enter his thoughts. When your ego is provoked, the entry point for Satan opens up. The window for Satan is flung open. When your ego is provoked, you give an entry point to Satan. If you become modest and your conscience is awakened, you give entry point to angels. Angels enter through your conscience, and Satan enters through ego. Zoroaster had said: The world is a perpetual battle ground between good and evil force. The truth is that there are two different creatures: angels and Satan. Satan wants to deviate a person and angels want to bring a person on the right track. I will give an example to explain ego and conscience. 
The Prophet had a Companion called Abu Bakr. 

A person began to speak in a bad way with this Companion of the Prophet. He tried to provoke the Companion through his words. First Abu Bakr kept listening silently. When the person spoke too much, Abu Bakr became provoked and reacted in return. When Abu Bakr reacted, the Prophet got up and left. Abu Bakr later said to the Prophet that when that person was speaking badly, you stayed there. When I said something, you left the place. The Prophet replied when you were silent and he was speaking, an angel was present who was speaking on your behalf. You were silent, but the angel was speaking on your behalf. When you began to speak, Satan arrived at the place.When Satan came, I saw no reason to stay and so I left. (Musnad Ahmad 9624)

There is nothing mysterious in this. When Abu Bakr was silent and the angel was speaking, what did this mean? The angel was activating the conscience of the other person. The angel was telling him that you are doing something wrong. On the one hand, the angel was highlighting Abu Bakr's dignity: "He is a person who is silent and isn't reacting. You are so wrong in speaking badly with him." The angel had a chance to dignify Abu Bakr and show that the other person is morally low. The person's conscience then began to criticize him. When Abu Bakr was silent, the other person's conscience was telling him that he was doing wrong by speaking badly with an innocent man. Angels activate us just as Satan activates us. When Abu Bakr was silent, the angel was activating the conscience of the other person. But when Abu Bakr began to speak, Satan got a chance. The window for Satan's entry opened up. The work being done by the angel stopped there. This is the meaning of the above hadith.