All of the answers are in Allan Kardec’s The Spirits Book, but for those who like to pursue a subject at a time, I am reprinting the pertinent information here. Here is the broad summary:

“We may place in the first or highest rank those who have reached the degree of relative perfection which constitutes what may be called ‘pure spirits.’ We may place in the second rank those who have reached the middle of the ascensional ladder, those who have achieved the degree of purification in which aspiration after perfection has become the ruling desire. We may place in the third or lowest rank all those imperfect spirits who are still on the lower rungs of the ladder. They are characterized by ignorance, the love of evil, and all the low passions that retard their progress upwards.” Read More.