Ascribing a particular definition to a demon tool is rather difficult, since some are simply machines whereas others are sentient beings. The best I can say is that they are an artifact that somehow meshes science and magic to achieve things otherwise impossible. Most were created by a man named Eibon.
What are some demon tools?
The first and most important is a machine we are all familiar with--BREW. Others include the Morality Manipulation Machine, and Weapons themselves fall under this category. There are a number of others, but as of now I am not aware of them.
Who is Eibon?
This is a question I cannot answer, there is very little information on him other than he was a scientist and friend of the deceased Shinigami.
What is BREW
BREW is the most powerful of all the demon tools, and currently in Shibusen's possession. It has the capability of granting one wish, no matter how great. Unfortunately the ways it does so can be rather roundabout, and it can only grant one wish at a time which is why we're all here in this situation. Shibusen occasionally holds BREW viewings so that you can see it for yourself.