Grímnismál stanza 20 contains four lines that are more unsettling the longer you look at them.
Huginn and Muninn fly every day over the wide earth. Odin says he fears for Huginn that he will not come back — but he fears more for Muninn.
Thought and Memory. He is more afraid of losing his memory than of losing his mind, and the god who says so is the one who traded an eye for a drink of knowledge and hung himself on a tree to get the runes. The stanza is spoken by Odin in the middle of a poem in which he is disguised, starving, tortured between two fires, and reciting cosmology at a boy — and the wobble in it is the only moment in the entire corpus where he sounds frightened of something other than the wolf.
The names
Huginn is from hugr, which covers mind, thought, mood, courage and intention — a broader word than English "thought," closer to the whole interior faculty. Muninn is from munr, memory and desire, and from the verb muna, to remember.
Both are ordinary Norse nouns given a masculine suffix, which is a common way of forming a name. Whether the ravens were ever anything other than personified faculties is unanswerable; Snorri, unusually, notices the problem and effectively says so, explaining in Gylfaginning that Odin is called hrafnaguð, raven-god, because he sends the two out at daybreak to fly over the whole world and they return at breakfast-time, so that he learns much news — and that this is why he is also called Thought and Memory.
The daily circuit
The picture is of a surveillance system with a schedule. They leave at dawn, cover the world, return in the morning, and sit on his shoulders and tell him what they saw. Skáldskaparmál and the skalds use them constantly in kennings, and a warrior "feeds the ravens" when he kills.
That last usage is the important context. Ravens follow armies, and they follow them because they eat the dead. A god of the slain who is attended by two of them is not primarily a figure of wisdom in that image — he is the presence that arrives after the fighting stops.
The Third Grammatical Treatise, a thirteenth-century Icelandic work on language, quotes an anonymous verse describing Odin sending them out and receiving them back, independent of Snorri and Grímnismál.
What the archaeology shows
Birds beside a helmeted figure appear repeatedly in Migration-period and Vendel-period Scandinavian metalwork. Helmet plates and pressed-foil figures from Swedish sites, notably the Vendel and Valsgärde boat graves, show a rider or warrior with birds at his head or shoulders, sometimes with one eye emphasised. Similar imagery appears on the Sutton Hoo helmet's fittings and on gold bracteates.
None of these are labelled, and identifying them as Odin with Huginn and Muninn is inference — but the combination of one-eyed figure, spear, and paired birds recurs often enough across several centuries and countries that the figure being depicted is very likely the same one the later texts describe. This is some of the strongest evidence that Odin's cult and imagery are much older than the Viking Age.
The raven banner is real and separate. The Anglo-Saxon Chronicle records the capture in 878 of a banner called Raven from a Danish army; the Encomium Emmae Reginae describes a banner of plain white silk on which a raven appeared when war was coming and hung still in peace; Orkneyinga saga has Sigurðr Hlöðvisson's mother make him a raven banner that grants victory to the host it is carried before and death to the man carrying it. Three of his standard-bearers die at Clontarf and Sigurðr eventually carries it himself and is killed.
Two ravens, two wolves, two brothers
Odin's attendants come in pairs throughout: the ravens Huginn and Muninn, the wolves Geri and Freki — "the greedy one" and "the ravenous one" — whom he feeds his own meat while living on wine alone.
Both pairs have transparent, near-synonymous names of the same formation. It looks like a deliberate poetic pattern rather than a set of characters, and it may be that the wolves and ravens are the same idea in two species: the animals that arrive at a battlefield, attached to the god who caused it.
Two birds as visual shorthand
The ravens are among the most reproduced Norse images in the world: tattoos, logos, jewellery, the badge of more than one military unit, and a persistent presence in every retelling from Marvel to God of War. Two ravens flanking a figure is now a visual shorthand for "Norse" in graphic design generally — see symbol traditions.
The Tower of London's ravens are frequently connected to Odin in popular writing. There is no medieval evidence for that at all; the Tower's raven tradition is not documented before the nineteenth century.
For Odin himself, see his entry; for the horse he rides on the same errands, Sleipnir; and for the Norse overview.
Are the ravens real birds, projections, or parts of Odin?
The sources let you have all three and never choose. Grímnismál has them as birds that fly and might not return; Snorri glosses them as the reason Odin bears the names Thought and Memory, which reads as personification; and the seiðr practices attributed to Odin elsewhere include his body lying as if asleep or dead while he travels in the shape of a bird or beast — described in Ynglinga saga. If that is the frame, then sending out Thought and Memory is a description of the practitioner's mind leaving the body, and the fear of them not returning is a fear about not coming back.
Do the ravens survive Ragnarök?
No text says. They are not listed among the survivors, not described dying, and not present at the final battle — where Odin rides out with the einherjar and is killed. Their absence is total after the eschatological material begins. Given how the corpus treats them as faculties as much as creatures, the neatest reading is that Thought and Memory do not outlive the head they belong to, but that is interpretation, and the poems simply stop mentioning them.