Magic (sorcery, dweomer, voodoo, witchcraft, The Force) is a supernatural force that allows those who are knowledgeable in the mysterious arcane arts to manipulate reality. This is the staple of fantasy fiction, but has been known to cross into other genre. Magic can be handled in different ways, based on the theme of the fantasy.
Magic in this genre tends to be flashy, potent and highly useful. Magical effects can range from simple twists of fate to objects cast in raw magic (magic missiles, floating disks) - anything is possible! Spells generally requires the caster to chant or resit a poem, make jesters, and/or possess some sort of object (foci, fetish, sacred object). Magically enchanted objects range from simple single-use effects (like a cape that make the wearer become invisible) to fully conscience relics (like an intelligent sword with an array of powers, and the ability to communicate). Most magic items are useful, but some can be cursed or out-right evil.
Powerful supernatural entities would give-out a part of their power to those who worship them. These beings are rightfully called Gods. Gods form into groups called pantheons, and each God or Goddess has a particular control over nature (weather, seas, volcanoes), human behavior (law, love, craftsmanship) and even reality (time, luck, fate). Gods can be good or evil, and powerful demons rarely achieve their power - being marginalized to cult-worship.
Sword & Sorcery
Magic in this genre tends to be sinister, dangerous and vary elaborate. Magical effects tend to be more subdued then High Fantasy effects, and avoids raw-magic, life restoration, alignment-based effects (smite evil, know alignment) or those that give-out all the answers at once (read magic, mind-reading). Curses, divination, mind-control, demonic summoning, and necromancy are the typical spells seen in this genre. Spells usually require long elaborate rituals, and maybe some sort of sacrifice (from self-infliction, to a virgin sacrifice). Magic items are not as common as their High Fantasy counterparts, but usually maintain the feel of the genre.
Gods tend to be vary subdued in this genre. They can ether be a well-believed hoax, or so dysfunctional, its just safer not to put a lot of faith in them. Because of the moral ambiguous nature of this genre, gods are generally not defined by good or evil - the difference between a ruling god and a Demon Lord is based on how mortals relate to them.
|"Any sufficiently advanced technology is indistinguishable from magic."
- Arthur C. Clarke, Third Law of Prediction
This is when you have science so advanced, that to all intents and purpose, it seems like magic to both the players and the characters. This makes sense when you have technology that can project holograms and force-fields, replicate and recompile matter, and materialize matter from elsewhere or out of nowhere. Leaving such technology as an ambiguous science allows a Game Master to convey a world where the characters are in a primitive state, while dealing with technology beyond their understanding, so the players don't have to suspend their disbelieve while the characters come to grip with the strange world they living in.
Star Trek is the best place to find such super science, with their teleporters, force-field screens, gravity-control, tractor beams, holodecks, matter-replicators, and so on. Nano-machines found in 6th edition Gamma World was designed to work like spells found in fantasy games.