Satan is a technically job title not an entity strictly speaking. With that said…
A Satanist is usually defined as a person who believes in, worships or follows after the example of or manner of a Satan (accuser/adversary) against or in opposition to restrictive societal norms values and established religious viewpoint.
It can take many forms. From acausal Chaos gnostic to hedonistic humanistic atheistic LeVeyan Satanism. There are probably as many kinds of Satanism as there are Satanist’s and that’s not even touching on divergent or variant forms like Luciferianism.
Basically though it’s a point of view and sometimes lifestyle that the common herd of humanity eschews because they’ve been taught to fear and avoid it.
It requires a questioning mind one that in the face of established norms goes “WHY?” or “WHY NOT?” and then proceeds to make up their own mind on normative values such as right and wrong as to what is right for them or what they feel is wrong for them and therefore is a line they won’t cross.
It’s a position most view as subversive evil or abnormal but it’s merely the position of a rational questioning mind that isn’t satisfied to just do as told but wants to know why.
It’s the same mind set as the scientist who struggles to know why gravity works or why the moon doesn’t crash into the earth.
But because it doesn’t toe the line and conform to societal norms without questioning them it gets labeled evil or subversive and a few people decided to relish in the fear and discomfort and capitalize on it and called it Satanism.
That’s my own UPG on the subject more or less though my take isn’t uncommon.
I suggest however you read up on all the variants of it and then see what you think defines Satanism and it’s difference from other beliefs.
It’s way too broad a subject for a forum post to do Justice to. It would take tens of thousands of posts to even begin to do Justice to the subject because every Satanist has their own view on it.
I’m not even gonna go into the divisions between theistic and non theistic types or you’d be reading my upg for hours so… that’s my upg (and I’m case you wondered I consider myself a a theistic Luciferian but I don’t worship or fear Lucifer or kow-tow to him like Christians do to their god. They’re taught do what I say or I’ll burn you when I destroy the world and that you must fear me because I’m all powerful and all knowing about their Christian god. I grew up Catholic I know that’s the gist of the teachings)
and that concludes my own take on the subject aka my UPG.
Add: it’s UPG partly because there is no possible way for me to interview or talk to all the satanists in the world then condense all the disparate views into a synopsis.