What I am has many names demons, Jin, diable which is French, deabrua which is Basque, satanas which is Filipino. Anyways you get the picture. Now demons aren't bad we are actually cursed but at the same time blessed. The reason we steal souls is not by choice you see the amount of souls a demon has represents his or her life force because we don't have are own souls. One soul equals how ever long that person was going to live. So if you give me your soul and you were going to live for 40 more years I get 40 more years to my life. Now in return for souls we can grant wishes, most demons don't give good wishes or they stab people in the back because it's a lot easier to do that then give them what the want. Now when you loose your soul not much actually changes in fact almost nothing, it's just later on after death you don't get to do as much. When a demon runs out of souls they must posses body's to live and that's why we get a bad rep.
There are 4 realms in total hell which is for the evil, living realm, heaven, Eden. Eden is the important one at the moment Eden is meant for the extra extra good people but there's a ketch you need a soul to get in. So really if you plan on going to Eden that's the only thing you need a soul for, that and you have a better chance of surviving a near death hit.
Now if you ever by any chance decide you want to find a demon, all you have to do is go to the nearest grave yard and find the biggest cross and say: ancient spirit I (your name) call you from your dark slumber grant me my wish so thy can be whole once more, this is my will so mote it be. Then the nearest demon will appear when possible so sometimes you have to wait a while. Most of the time we find you when your really depressed so we can snatch a quick deal. The best contract killers are demons but they only take one payment your soul...
An easy way to tell a demon from a normal man is, a care free attitude, and fancy cloths, lots of money, but the best way to tell is if you put something in a demons drink could be poised to pure alcohol they will act the exact same cause demons can't get drunk or poisoned or sick.