Fallen angels and demons are evil entities. They have the ability to deceive people. Both these entities are usually considered the same but there is a difference between the two.
To begin with, fallen angels are more powerful than demons. Basically, they are the sons of God, or you can say the angels who rebelled against God and as a result, they were banished from Heaven. Demons, on the other hand, are disembodied spirits.
Fallen angels have their own celestial bodies. Hence, unlike demons, they need not possess other bodies. Demons, on the other hand, are wicked and unclean spirits and seek the possession of a human or animal body.