Inner, inward, inside, interior, internal, intestine are comparable when they mean being or placed within something.
Although in many cases interchangeable, they are more or less restricted in their applications and therefore clearly distinguished in their implications.
Inner typically applies to something far within or near the center while inward typically applies to something directed within or toward the center. Both words apply also to the mental or spiritual, frequently with the added implication of something intimate, secret, or inaccessible.
Inside is used chiefly of spatial relations, but it may be used with reference to persons who are so placed in their work or who have such contacts that they may be said to be figuratively inside a place or group; thus, inside work implies a contrast with field or road work; inside knowledge of a negotiation implies participation to some extent in that negotiation.
Interior and internal usually suggest more abstract or technical and less intimate relations than inner and inward.
Interior frequently implies contrast with the outer limits of the thing itself; thus, the interior features of a country are by implication opposed to those of the coast or boundaries; interior decoration deals with the decoration and furnishing of the inside of a house or other building rather than with its outside; one’s interior life is one’s life as expressed in thoughts and aspirations rather than in outward activities.
Internal implies contrast with something beyond or outside of the outer limits of a thing; thus, internal evidence of a poem’s authorship is gained from a study of the poem itself rather than from outside sources; the internal affairs of a country are its domestic, as opposed to its foreign, affairs; internal medicine is that branch of medicine dealing with the diagnosis and treatment of diseases affecting the internal organs (as the heart, lungs, stomach, and liver).
Intestine is a close synonym of internal used specifically of what may otherwise be described as domestic or civil (as opposed to foreign ) with, however, the connotation of an evil or mischievous origin or nature.