A persona, in the word's everyday usage, is a social role or a character played by an actor. The word derives from the Latin for "mask" or "character", derived from the Etruscan word "phersu", with the same meaning.
Criticism of poetry and fiction refer to a "second self" created by the author and through whom the narrative is related. Importantly, attributes and attitudes associated with the persona are understood to be separate from authorial intentions, per se, though there may in fact be some overlap between the two. For instance, in Dostoevsky's novel, Notes from Underground (generally considered to be the first existentialist novel), the narrator ought not to be conflated with Dostoevsky himself, despite the fact that Dostoevsky and his narrator may or may not have shared much in common. In this sense, the persona is basically a mouthpiece for a particular worldview. Another instance of this phenomenon can be found in Brett Easton Ellis' novel, American Psycho, the story of a sociopathic murderer living in New York City, who is a successful, if very troubled, Wall Street executive by day. The work is one of social satire, and as such may well reflect a good deal of authorial intention, but the persona of Patrick Bateman (the novel's first-person narrator) ought not to be conflated with the novel's author.
In both of the examples just given, the persona is an active participant in the story he is narrating — it is his own story — but this need not be the case. To take another example from Dostoevsky's work, the narrator of The Brothers Karamazov is not an active participant in the story, but nevertheless presents a clear perspective on the events concerned therein. In other words, the invisible and omniscient narrator of Dostoevsky's novel gives the reader the impression of taking a definite attitude toward the proceedings being related, albeit subtly so, and mainly by tone of description and idiosyncratic phrasing.
Finally, the twentieth century has provided us with many intermediate instances. One example is Faulkner's novel As I Lay Dying, a story told entirely via the interior monologues of fifteen different first person narrators, and thus from the same number of differing perspectives. Another example of a vague or undefined relationship between narrator, protagonist, and persona--perhaps the preeminent such example in the English language--can be found in James Joyce's novel, Ulysses. Here we find instances of direct first person narration, third person narration mixed with first person stream of consciousness, dozens of pages of catechismic question-and-answer, a surrealistic stageplay-like episode with dialogue and stage directions, and finally the famous extended first person stream of consciousness soliloquy that closes the book. Examples such as these tend to blur or call into question the role of a persona, at the same time as they supply rich fodder for academic analyses of the works themselves.
To sum up, a persona can, broadly-speaking, be understood as the "organizing consciousness" of the narrative. This clearly differentiates it from any characters, even major and well-developed ones, who do not steer the reader's perspective on the proceedings. However, in some very well-defined cases, the question might arise: Why bother positing an organizing consciousness, understood on some level to be separate from that of the author, at all? Different schools of criticism will have differing answers to this question, and some — the post-structuralist school, for instance — might take issue with the very notion of a single organizing consciousness. But in general, the practice is adopted as a handy way of understanding the guiding principles of a work without treading too far into disputes about what a particular author was "really like" or "really thought about things" in his or her own personal life.
Charles Dickens and William Blake, for instance, were widely known to have progressive attitudes regarding the difficulties faced by the working classes in Victorian England and the effect of England's industrial revolution on contemporary life, respectively, and their attitudes were clearly reflected in their work. But other cases are not so clear-cut. Very little is known about the life of Thomas Pynchon, but his books, in particular, Gravity's Rainbow, have achieved iconic status in modern Western literature. Furthermore, if the interpretation of a work is taken to be fundamentally the process of deciphering an author's personal feelings about various subjects — an attempt to understand the mens auctoris (mind of the author) — then it might be argued that literary criticism thereby degenerates into a kind of pseudo-psychoanalysis, leaving little room for consideration of the works themselves. Finally, and for similar reasons, the narrator-as-persotation allows for greater interpretive latitude, and thus arguably richer interpretive possibilities, than a more strictly authorially-centered approach might.