Angels are not all the same, some are more powerful than others and are specialised by the Creator for different tasks. Each rank of angels has an infinate number of places open, apart from Archangels who are the most powerful angels.
Not to be confused with the baby-like angels that they are often portrayed as in the media of today, Cherubs are angels who have legs and wings much like a giant eagle, the feet ending in sharp talons when in flight. The legs of a Cherub can change from those of an eagle to those of a human almost instantly. Cherubs are the weakest type of angels and act as the guardians of Heaven's outer reaches.
Seraphs are what make up the bulk of the armies of Heaven. Every Seraph has six wings and are physically stronger than Cherubs, notably more human-like in appearance. Often armoured, the Sepahim can found within the middle layers of Heaven when not following orders from their commanders, the Archangels.
Thrones are angels who are created by the Creator in order to observe the mortal world. While they do this they also gather intelligence on demons and their activities both in the world of mortals and within Hell. Usually used as support for the armies of Heaven but can also be seen as spies. They are not made for direct combat.
Archangels (Only Nine);
The Archangels are angels who have been lifted higher by the Creator, granted extra strength and given command over the armies of Heaven. When one is killed another angel is selected and elevated by the Creator to serve as the Archangel who is being replaced. The command of an Archangel is second only to the command of the Creator.