The Bible doesn't have a lot to say about demons, although Jesus is recorded as having cast demons out of people, demons which made them ill or crazy. But there was plenty there for medieval people to build on. Demons were usually invisible, but they could take visible shape, such as like a black dog or other animal. A knight riding to commit a foul deed might abruptly discover that what he thought was a horse was actually a demon, planning to toss him in the nearest river.
But most commonly demons were depicted as vaguely human, usually with horns, sometimes with tails. One of the things demons loved best was hurting people, so carvings on churches often depict them doing so with great gusto.
The above grinning demon is from the eleventh-century monastery of Tournus.
Some of what were described as "temptations" were actually very painful. I guess the idea was that people would promise the demons anything to make them stop. Below is a late medieval painting of the "temptation of Saint Anthony." The demons are having a blast, but Anthony is clearly not. I used this image for the cover of my fantasy novel, "Is This Apocalypse Necessary?" which (among many other things) includes demons.
Medieval people didn't really make deals with the devil in a nineteenth-century, Faust kind of way, because they realized how little of value the cruel demons could actually offer. Instead, demons stood by, ready to take souls away to hell, as in the below image of a rich, dying miser whose soul is being snatched while his moneybags are useless below his bed. (It illustrates the story of Dives and Lazarus.)