Elemental is a creature type.
They are the incarnations of the very elements that make up the multi-universe. They appear in just about every set. Elementals come in every color. The color, ability and strength are determined by the name and elemental. For example, Spark Elemental is a spark with properties similar to electricity and dissipates into nothingness at the end of the turn.
These are red elementals that appear in ravnica and lorwyn. They are humanoid elementals that are filled by flames. The Flame-kin pilgrim, Ashling (Ashling the Pilgrim) is a burning spirit with a direct connection to the elemental world. When the aroura came, it transformed her into Ashling, the Extinguisher and all the flame-kin into the new blackened elementals called Cinders.
The other elementals of Lorwyn. They are creatures and instants. They have the Evoke ability: (You may play this spell for its evoke cost. If you do, it's sacrificed when it comes into play.) And an ability that resolves when they come into play, or when they're put into the graveyard from play.
In the Unhinged set, there is an elemental called "Question Elemental?" which requires both players to speak in questions. When a player makes a statement, rather than a question, the other player may take control of Question Elemental?