Fiend
In D&D and Pathfinder, a fiend is a negative supernatural entity—what we would generally call a demon.
The term is a bit weird since, again, fiends are what we'd call "demons." The term demon, however, is reserved in D&D and PF for one specific type of fiend: a Chaotic Evil race hailing from the Abyss.
Therefore, Asmodeus by this definition would not be considered a demon. He'd be a fiend—specifically a devil (a Lawful Evil fiend). This is especially weird since Asmodeus IRL is referred to as a demon.
To make matters even more confusing, you may hear various NPCs (and even me) referring to all sorts of fiends as demons. Fun.
Main types
The three main types across both D&D and PF are devils, demons, and yugoloths (aka "daemons"). These three groups align with the three Evil alignments: devils are Lawful Evil, demons are Chaotic Evil, and yugoloths are Neutral Evil.
Basically, in ascending order of "how badly do I NOT want to run into one of these guys," it goes: devils, yugoloths, demons.
