Although the Aeneid shares many characteristics with the Homeric epic, as an epic it is different in important ways. For this reason, the Aeneid is referred to as a literary or secondary epic in order to differentiate it from primitive or primary epics such as the Homeric poems. This, should not be interpreted as value judgments, but merely as indications that the original character of the epic was improvisational and oral. Aeneid, composed later in the epic tradition, was basically non-oral and crafted. The Homeric poems give evidence of improvisational techniques of composition involving the use of various formulas. This style of composition is suited to the demands of improvisation before an audience which do not allow the poet time to create new ways of expressing various ideas. In order to keep his performance going he must depend upon stock phrases, which are designed to fill out various portions of the dactylic hexameter line. On the other hand, Vergil, composing his own personal poetic language. Thus, in the Aeneid, absence of the continual repetition of formulas can be seen, which are unnecessary in a literary or secondary epic.
Virgil, however, does imitate Homeric language without the repetitions. This is another reason for calling the Aeneid a secondary epic. For example, Virgil occasionally translates individual Homeric formulas or even creates new formulas in imitation of Homer such as 'pious Aeneas', imitates other Homeric stylistic devices such as the epic simile and uses the Homeric poems as a source for story patterns. Although in this sense the Aeneid can be called derivative, what Virgil has taken from Homer he has recast in a way which has made his borrowings thoroughly Virgilian and Roman. Virgil changed the value system characteristic of the Homeric epic, which celebrated heroic individualism such as displayed by Achilles in the Iliad. The heroic values of an Achilles would have been anachronistic and inappropriate in a poem written for readers in Rome of the first century B.C., who required their leaders to live according to a more social ideal suited to a sophisticated urban civilization. Therefore, although Virgil set the action of his poem in a legendary age contemporary with the Trojan War before Rome existed, one must judge the characters of his poem by the standards of the poet's own times.
The Aeneid differs from the Iliad and the Odyssey in that it often gives evidence of meaning beyond the narrative level. Homeric narrative is fairly straightforward; there is generally no need to look for significance which is not explicit in the story. On the other hand, although Virgilian narrative can be read and enjoyed as a story, it is often densely packed with implicit symbolic meaning. Frequently the implicit reference is to Roman history. While Homer is little concerned with the relationship of the past to the present - the past is preserved for its intrinsic interest as a story - Virgil recounts the legend of Aeneas because he believes it has meaning for Roman history and especially for his own times. For example, the destruction of Troy resulting in the wanderings of Aeneas and his followers west to find a new life can be seen as parallel to the history of Rome in the first century B.C., which included both the violent destruction of the Republic and the creation of peace and order by Augustus. Also suggestive of the Roman, 'civil war' between the Trojans and their Italian allies, and Aeneas's victory over the Italians in this war suggests Augustus's ending of the Roman civil wars. It shows the importance of Roman history in the Aeneid.
Another important difference between the Aeneid and the Homeric poems is that the former has a philosophical basis while the latter were composed in an era completely innocent of philosophy. The Aeneid gives evidence of the influence of Stoicism, a Hellenistic philosophy which had gained many adherents in the Greek world and by the first century B.C. had become the most popular philosophy of the educated classes at Rome.
Anchises's digression on the nature of the universe and human existence (6.724-751), which combines Stoic physical theory with Orphic and Pythagorean teachings (transmigration of souls). On the other hand, the gods in the Aeneid for the most part do not reflect the Stoic view of divinity. They are basically the traditional anthropomorphic deities of myth as required by the conventions of epic. However, Stoic influence is evident as in book 1 when Jupiter is closely identified with providential fate.
In the Aeneid there are innumerable echoes of the Iliad and the Odyssey. Perhaps, in book 12 where your knowledge of the Iliad will enable you to see how important figures of Vergil's poem are associated in various ways with heroes of the Iliad. These references to the Iliad provide an interesting and significant commentary on the action of the Aeneid.
Finally, one should be conscious of recurring images in the Aeneid such as snakes, wounds, fire, hunting, and storms, and their meaning for the narrative. As an imagery, the Virgilian technique of making a real part of the story an image and vice-versa, can be noticed. For example, consider hunting in the Aeneid. In book 1 Aeneas is a real hunter who slays deer; in book 4 in a simile he is a metaphorical hunter of Dido and then again, a real hunter as he and Dido engage in a hunting expedition. No doubt Virgil intended these three instances of hunting to refer to each other implicitly and to comment upon the story. Recurring words have a significance in the Aeneid uncharacteristic of the Homeric poems, which, due to the nature of oral poetry, as a matter of course employ constant repetition of formulas. Two of the most important recurrent words in the Aeneid are furor, which means `violent madness', `frenzy', `fury', `passionate desire' etc., and its associated verb, furere `to rage', `to have a mad passion'. These words have important meaning for the characters in the Aeneid to whom they are applied and whose behavior must be evaluated by reference to the Stoic ethical ideal. In addition, these two words connect the legendary world of the Aeneid with Roman politics of the first century B.C. because they were often used in prose of the late Republic to describe the political chaos of that era.