The Salem Coven (and Adrian) are the only witches with elemental abilities, due to them being descended from the first coven of witches to bind their powers with nature. All members of the bloodline have the potential to inherit any of the four elemental abilities, their powers usually reflecting their personalities. At any one point there are two of each element (one boy, one girl) in the coven.
Witches of the bloodlines who are not alligned with the Salem coven are not bound by this and can have any of the four powers. An example of this is Adrian Kristoff, he is not a member of the Salem Coven and so there are currently three active pyrokinetics whereas the other three elements only have two.
Pyrokinesis is the power to create and manipulate fire.
Aerokinesis is the power to create and manipulate wind.
Hydrokinesis is the power to create and manipulate water.
Geokinesis is the power to create and manipulate earth.