Basic Information.

Who are they?
Elemental Alchemists are people who control elements, who do things with the elements. They can do many things with the elements. They are researchers, trying to find secrets in the elements, which is a very vague domain. Many humans do not know that they exist, because alchemists normally stay locked in a dark place, thinking to themselves, because they need a high rate of silence to get their concentration.

What differs them from humans?
Nothing. Elemental alchemists are humanoids, like you and me, hopefully. The only signs that they are alchemists is that they can control the elements, and they normally have pale skin, but that is just a generality among the alchemists.

How do people become alchemists?
To become an alchemist, one must read the book of elements very carefully, before even trying to think that they will be alchemists. Elemental alchemists seem very diverse, but you'll see that every alchemist has great concentration, and the will to work, even though most don't cooperate with each other, only working for themselves.