The elemental forces are a metaphysical or poetic division of reality very different from the atomic elements and fundamental forces identified by modern science. In popular fiction, they are almost always associated with fantastical magic or the supernatural.
In general, the term "elemental forces" usually refers to characters or concepts connected to the four Classical Elements from the Greek tradition later adopted by Medieval alchemists: fire, earth, air, and water. Sometimes, a fifth element is added to this group; this element varies from one fiction to other. In the alchemic tradition, it was called Quintessence (which means literally "fifth element"), an element capable of changing from one to another, or Aether, the element along which light and sound travel the way that leaves might float down a river. In other stories, the fifth element is identified as the essential void or shadow or else identified as the soul or lifeforce. In the movie *The Fifth Element*, the last element was an emotion, love, whereas in the TV series *Captain Planet*, the fifth element is heart.
In popular fiction, the notion of the Classical Element has become fused with the Aristotelian notion of the metaphysical ideal (i.e. the purest form of something). Thus, popular modern derivatives of these Classical elements include lightning, ice, smoke or fog, gem or crystal, radioactivity, death, time itself or aging, and in superhero comic books even "elemental pollution" and "the quantum elemental force".
The term "elemental power" usually refers to any character with control over one of these elements and/or their derivatives, with these elements as their power source, or with a body made from one or more of these elements.
While the Greco-European tradition has four elements, the Chinese-Asian tradition has five traditional elements: water, fire, wood, stone, and metal. Just as the Greco-European tradition often includes an honorary fifth element in popular fiction, most often the soul/heart or shadow/void, the Chinese-Asian tradition often includes an honorary sixth element in anime' and video games, most often lightning, light, or ice.
Superheroes who seem to follow the tetrapartite or fourfold elemental schema include The Fantastic Four (The Human Torch = fire, The Thing = earth, The Invisible Woman = air, and freeflowing Reed Richards = water) and many incarnations of The Power Rangers.
Superhumans who wield specific classic elemental forces include DC's Firehawk and Sunboy and Marvel's Firelord and Sunfire (fire), DC's Terra and Stone Boy and Marvel's Sandman and Terrax the Tamer (earth), DC's Red Tornado and Typhoon and Marvel's Whirlwind and Turbo (air), and DC's Mera and Marvel's Hydroman (water); if one includes the Chinese-Asian elements, one can include DC's Chlorophyll Kid and Marvel's Plantman (wood or plant) and DC's Ferro Lad and Marvel's Colossus (metal).
Superhumans who wield derivatives of the classic elemental forces include DC's Lightning Lad/Livewire and Marvel's Electro (lightning), DC's Shadow Lass and Obsidian and Marvel's Cloak and Darkstar (shadow), DC's Killer Frost and Mr. Freeze and Marvel's Ice-Man (ice), DC's The Ray and Doctor Light and Marvel's Dagger and Dazzler (light), DC's Sonar and Marvel's Banshee (sound or vibrations), and DC's Brother Nature and Marvel's Thor and Storm (weather or storm).
In terms of superhumans who have fused the notion of elemental forces with the Aristotelian notion of metaphysical ideals, the extreme in American comic books would be the reboot of Brother Power as a "doll elemental" (technically an elemental of all effigies, including dolls, puppets, statues, and animatronic figures), while the extreme in Japanese manga would be The Paper, a bibliophile who can control paper in any form.