(This post is part of the "7 things you ought to know about the devil.")
Dualism is the belief that there are equal and opposite forces that push against each other. In some ways it can be like the yin and yang of the Eastern religions, but the idea is more like the pagan concept of a good god and an evil god in constant war against each other.
This pretty accurately describes the way most Americans, even Christians, think of the devil.
Look at popular culture (from Southpark to Spawn) and you'll get a picture of two kingdoms locked in pitched battle each hoping that they'll be able to turn the tide and defeat the other side. The technical term for this is "horse hockey."
The biblical picture is far more one-sided. Yes, we see that the fighting is fierce, and the angels must occasionally go toe-to-toe (see Dan 10:12-14), but there is One whom the demons fear (see Luke 8:26-31) and His victory is certain (Rev 19:11-20:10).
Contrary to popular culture, the spiritual war is not two equally matched sides. It is one all-powerful side patiently exchanging a few spit balls with the enemy until the time comes to drop the nuclear bomb.