Obviously, it's going to vary broadly per individual even within denominations, but you can get an idea of the various stances out there by looking at the differences between denominations.
Biblically, demons are the fallen angels, those angels that sided with Satan against God. Demons represent the will of Satan the same as angels represent the will of God, so it is reasonable to think of them as coming to Earth on occasion (or even frequently), since it is thought that God, Satan, and angels do so as well. In contemporary thought, demons and angels are present as disembodied forces that are likely all around you, but that you may not ever be witness to (Hollywood simplifies this even further by depicting them as bodies that are invisible to mortals in movies like in City of Angels and Constantine). Most Christians will come down somewhere on the spectrum between "demons/angels are a metaphor for forces that are beyond an individual's control" and "they're actual beings that wander around and still do things just as they always have".
But when asked, they'll usually cop out with something trite like - "I dunno. It's possible - God works in mysterious ways!" An unfortunate majority don't know what they believe.
Edited 2 time(s). Last edit at 11/13/2012 11:51AM by Splntrd.