"Spell" is the general name given to any magic effect available to characters or creatures through the use of items such as scrolls, wands, staves, or spellcasting abilities. Spellcasters generally need resources to cast. Most often mana is required but sometimes other ingredients may be needed for spells to work or are included as amplification of effect.
Usually, spells are grouped into categories or magic schools. Commonly used categories are:
Effects involve healing, curing, creating protective auras, giving benefits to caster or target creature or object.
Effects involve destruction of, harming or weakening targets.
Enchantments, spells of nature, protection, and healing. Favored by priests, druids, and paladins.
Mostly elemental spells. Used mainly by wizards, elementalists and mages who prefer a more scientific approach to magic.
Violent and destructive area of magic concentrated on doing damage to everything that can be found in a dungeon. Most spells used for killing monsters belong to this category.
Calling demons, elementals or other creatures from various worlds and planes to serve caster temporarily. Summoned monsters are not necessarily friendly to the caster who called them to this world. Beings often disappear after the bond tying them to the current place fades, but some roguelikes allow them to stay indefinitely with the caster.
This subtle area of magic specializes in binding magical properties to items and creatures. Offers a range of handy utility effects. Enchantments alone rarely provide enough staying power for the caster to lead him or her to victory but may be a great boon when combined with other realms of magic.
Necromancy allows casters to raise slain foes as undead slaves. Some games define undead as a soul bound to animated corpse (or part of), therefore limiting necromancers' resources. Other games use systems in which only the corpse of formerly living creature is needed for these spells to work. Apart from creating slaves, necromancy allows for a different use of corpses and offers a palette of other effects closely connected with death. Some of them are very cruel.
A whole school of magic used to flee situations. Typical uses are to teleport away from monsters, to teleport the monsters away from oneself, or even to speed up oneself or slow down monsters in order to run away faster. Spells differ as to how far the affected objects will travel and to whether the caster can control the destination; some games even allow long-distance travel between dungeon floors. Some games permit the establishment of permanent portals between remote locations. In a few roguelikes, the hero can even summon remote objects or return fleeing monsters. The ability to teleport is so useful that non-mage characters will seek items that provide this capability.
Magic intended to reveal knowledge. Often used to locate monsters, objects, or traps, to see invisible things, to magically map the dungeon, to probe the status of monsters or the hero, or to predict the future fate of a character.
Magic dedicated to rapid changes and mutations. Used to warp one's body, polymorph creatures or items, transform obstacles. Commonly casters polymorph dangerous foes into other organisms to put them in advantageous situations or change their form to more powerful one to deal with nasty encounters. Casters mutating themselves take great risks but rewards are very tempting.
The typical use of an elemental spell is to attack a monster (as with conjuration), but the choice of elements is crucial to effectiveness against certain monsters. For example, it might be difficult to hurt a salamander with a fire spell, but an ice spell will do much damage.
Quite good for offense. Has means to slow not resistant targets.
Air magic spells can produce clouds which can be used to block monsters' LOS, to provide cover, poison or to create other effects that can be carried by gaseous substance. Among its destructive capabilities, lightning is especially worth noticing. Known to bounce off obstacles and strike nearby creatures. In some games flying targets are immune to electrocution because they are not grounded. Air magic often bears useful enchanting spells.
Many protective spells fall within this realm. Any offensive capabilities usually deal physical damage.