There are two distinct types of spirits of Athas: elemental and primal.
Elemental spirits are the essences of earth, air, fire or water (there are rumors of other, less basic types as well, such as silt or magma). They are the spiritual essences of the basic elements of existence, as manifested in sun, wind, rock and pool. As there are no alternate planes (that PCs are aware of, at least), elemental spirits do not come from the inner planes or the elemental chaos. Instead, they are the living embodiments of the physical elements themselves, able to inhabit, unseen, physical manifestations of the elements they embody, or manifest as elementals.
Primal spirits are fey spiritual manifestations of the life force of Athas. Primal spirits can be found in many types and sizes. Many creatures with the “fey” keyword are in fact primal spirits. Many take the form of animals or humanoids when corporeal. The most powerful (called Spirits of the Land) can be hard to distinguish from elementals on the rare occasions that they choose to manifest in visible form. Spirits of the Land are large, powerful spirits that inhabit particular geographic features, such as an oasis, a breezy canyon, or a rocky outcropping. Any primal spirit can become insubstantial and invisible (whether it can attack in this state is dependent on the individual creature description). It appears as a shapeless mist to creatures that can see invisible things. In this state, it can merge with the natural substance of Athas. Few people other than Primal Guardians (commonly known as druids among the people of Athas) ever see a primal spirit, and most Athasian city-dwellers view them as fairy tales.