There are many kinds of deities, but we can begin with the simplest. This kind of deity is called a demigod or a devata. It is an administrative position, as the president of a country. The position is permanent, but the person is not permanent. So, a different person can take the same position. Examples of demigods are Surya, Chandra, Indra, Yama, etc. They are the administrators of the universe. They occupy their posts for thousands of years, but they are not permanent. Just like in the government, there are different departments — finance, agriculture, commerce, education, health, etc. In the same way, these demigods are heads of these departments. Under these heads, there are also subordinates, which are also demigods but of a lower level. Just like a minister has a bureaucrat who executes the plans of the minister. So, there is a hierarchy of administration.
In Sankhya, causality has three parts — these are called bhautika or object, atmika or subject, and daivika or controller. When your eyes see an apple, the contact between the eye and the apple is arranged by the demigod. Not everyone can come into contact with everything. This contact is arranged by our karma and the demigods are responsible for delivering this karma. So, by our good or bad deeds, and the action of the demigods, we see what we see. The demigod or the controller is the person who arranges what types of objects come into contact with us.
There is another kind of deity, which is called Bhagavan or ‘God’. There are many forms of God as well; some are higher and some are lower. This ‘high’ and ‘low’ is simply the designation or the role that God is playing. For example, sometimes God appears to kill demonic people. This is considered a low-level role. The high-level role is when the same God plays with His friends or dances with His girlfriends. The ‘high’ and ‘low’ is the role, but since the same person performs this role, these deities are not considered different from one another. When you go swimming, you wear a swimming costume. When you go to work, you wear a formal dress. So, according to the work you are doing, you change your appearance. Like that God changes His appearance to perform different kinds of roles. But this is just a difference in role, not the person.
The term ‘angel’ can be employed both for servants of God, as well as for the demigods. Probably the most appropriate use of this term is as devotees of God because they transmit the message of God to common people. However, by that measure, there is no such thing as a ‘fallen angel’.
Angels play an important role in messianic religions, where one angel imparts the message of God. Angels don’t play an important role in Vedic religion because God speaks directly, not through an angel. Whenever there is a decline in religion, God appears to reestablish religion. So, even as angels or messiahs can come, they are not the only means to deliver the message. The message is directly imparted by God (in Abrahamic religions, God never appears in this world, and hence messiahs are the source of information about God). The Bhagavad-Gita is, for instance, spoken by God to His devotee Arjuna on a battlefield in front of everyone. It is not a private conversation and not a secret revelation. This is an example of why angels and messiahs have a secondary role in Vedic religion–the primary instruction is directly given by God.