Both spiritus and anima seem to have the definition of soul, but it is mentioned on numerous sites that they are different from one another. What is the difference?
The Roman religious principle was, peculiarly, not of worship but of consultation through proper ritual, of an unemotional compact with the gods, bringing to the Romans those senses of duty and security which, together with a fixity of purpose, became their distinguishing characteristics through the republican times and beyond. Words such as spiritus and anima are parts of a whole set of words from early Roman religious belief, which was basically ‘animist’, in that it recognised a spirit, anima, in every natural phenomenon.
The spirit of a stream, a tree and so on was anima. Anima did not refer to human life and was not what we might recognize as ‘soul’ (the latter is animus and was the essential principle of human life). Every tree and stream had its peculiar spirit, and unseen powers guided or helped the Roman and his family through life’s critical moments. Certain of these ‘spirits’ (as we might now confusingly call them) had their peculiar titles and duties — each household looked to its own Lares and Penates, who respectively watched over the dwelling and the grain-store, while Vesta was the goddess of fire and Mars (in the earliest times) the god who brought crops to life.
Spiritus was more tangible, the very ‘breath of life’, the actual ‘life force’, or ‘life principle’. It became symbolic of the spirit by which men might feel themselves driven, and thus the ‘mental spirit’ related to ardour and courage. It was later used for ‘spirit’, ‘soul’ or ‘mind’, and was close in meaning to mens, the active faculty of recollection, intellect and understanding.
Other words have similar uses, so that manes, phantasma, umbra, lemures, and larva (the latter two being malevolent) can all be distinguished in technical terms from spiritus and anima, according to context : apparitions or the shades of the dead, and so on.