Most Christians reserve the word "devil" for Satan (literally "the adversary"), and demons are his minions. The Revelation of St. John tells us that a third of the host of heaven followed Lucifer / Satan.
However, the New Testament uses the word "devil" in a more general manner, almost how we currently use "demon" (eg. an evil spirit). In fact, the Joao Ferreira de Almeida translation (Portuguese) uses the word "demonio" ("demon").
From where I see things, devil is the exact adversary of God. Devil is otherwise called Satan. Demons are those entities those are "negative" or "harmful" in nature. From the stand point of most Eastern philosophies, we do not have a "satan" or "devil" as the central controller of negative forces because as we see and experience, demons are creation of God as well. It is upto each one of us, with the wise use of choice to harness the "good" power of God and overcome the "demonic'.
Moon has correctly explained the views of Christianity. The devil and his demons play opposite to God and his angels. And according to the Revelation of St. John, even the devil and the demons were once counted among the hosts of heaven.