'Spirits' is a category for natural personifications of nature giving form by magic. It is said that worship of 'nature spirits' created most of the known spirits, providing the energy needed to give 'life' to an aspect of nature. That however is speculation as no one knows for sure.
While some can still be found in the mundane side of the veil (not being able to be separated from their place of origin), the low levels of energy in the mundane world since The Veil Treaty means that most of them are asleep. A few are lucky enough to live near ley lines and could be woken up under the right circumstances, both good or bad. It's common for the mages to keep an eye on these spirits.