These are creatures embodying the basic building blocks of the universe. In traditional western philosophy, this is recognized as being the elements of Sorcery - Air, Earth, Fire and Water. Eastern philosophy may replace Earth with Bamboo and Soil or use different elements altogether. Some schools accept Ice or other forces as elements.
What keeps elementals together is their strong ties to this world. Though there may exist elemental planes that these creatures can be summoned from, they are nevertheless a part of this world. They tend to lack the higher purpose of Entities and care more about worldly matters. Many elementals are simpletons or wholly incomprehensible.
All Elementals have the Limitations Elemental. Spirit and Supernatural Creature. They are fully affected by Summoning. In addition, they can be affected by elemental Sorcery. Some have more limitations than this, a fairly common limitation is Vulnerability to some other element.
Base Elementals
These are creatures, usually humanoids, formed out of a pure element. They commonly have DamageImmunity and ElementalAttack based on their element.
Air
Air elementals appear as cloud formations or pillars of barely visible churning air. As humanoids they are flighty and thin, often teasing.
The virtues of air are courage, passion, and inspiration. The vices are whimsy and rage.
Common Schticks include Amorph. They are tied to Air sorcery.
Earth
Earth elementals appear as moving rocks or pillars of earth. They can also manifest as a hole in the ground or a simple pile of dirt. In humanoid form they tend to be squat, even Oreids (mountain nymphs) are buxom.
The virtues of earth are fortitude, determination and stoicism; the vices are conservatism and selfishness.
Common Schticks include Armor. They are tied to Earth sorcery.
Fire
Fire elementals appear as creatures or pillars of flame. In humanoid form they move quickly and purposefully.
The virtues of fire are energy, thrift, and intelligence; the vices are destructiveness and arrogance.
Common Schticks include Blast. They are tied to Fire sorcery.
Ice
Not one of the common elementals, but summoned fairly regularly sorcerers in colder climes. Ice elementals appear as moving blocks or ice or animated icicles. In humanoid form they are rail-thin with sharp, angular features.
The virtues of ice are endurance, will, and perception; the vices are cold-heartedness and lack of care.
Common Schticks include Armor. They are tied to Ice sorcery.
Water
Water elementals appear as waves or pillars of water. They tend to take round, eye-pleasing forms. In humanoid form they tend to be extremely beautiful and effete.
The virtues of water are love, care, and empathy; the vices are sloth and indulgence.
Water elementals are tied to Water sorcery.
Wood
An Asian element, westerners see wood elementals as a subtype of earth elementals. Wood elementals appear as trees - from young saplings to ancient oaks. In humanoid form they tend to be long-limbed and willow.
The virtues of wood are patriotism, resilience and the ability to start over; the vices are xenophobia and callousness.
Common Schticks include Armor. They are tied to Nature sorcery.
Subtypes
Elementals come in many subtypes. Almost all of these exist in variants for each element, so a typical elemental has both an element and a subtype.
Element Creatures
These are people, animals and even plants imbued with elemental essence. A fire-hare or a water-oak. They appear and behave as the original in most respects, except where their elemental nature takes over. These creatures are common on the elemental planes.
Genies
Intelligent spirits of the elements common in Arabian myth, genies are like people, only more expressive and direct. Most are quite powerful and rule retinues of lesser elemental creatures in their home. Many have Wish.
Humors
Immaterial spirits and manifestations of the elemental virtues and vices.
Nature Spirits
Spirits of the natural elements, often communed with by shaman and Nature mages. These rarely take a specific form, and their powers tend to be subtle and unnerving. They need not conform to any specific element, and can represent a mixture or only a specific aspect, like thunder.
Nymphs
A type of nature spirits that take the forms of beautiful women. Nymphs are quite specific, there are not only water-nymphs, but actually different forms for different wells, for brooks, rivers, the ocean surface and the ocean depths.
A partial list of nymphs include; Nerids (wave nymphs), Oreids (mountain nymphs), Dryads (tree nymphs) and so on.