None of the earlier answers to this question have cited any authority for the particular distinctions that they make. In case some readers may be interested in reference-work assessments of the relevant differences, I offer discussions from several such works. I should note at the outset that none of them include coverage of overview, although I have no idea why they don't.
James Fernald, Funk & Wagnalls Standard Handbook of Synonyms, Antonyms & Prepositions, revised edition (1947), includes coverage of abstract, summary, and synopsis (along with abbreviation, analysis, compend, compendium, digest, epitome, outline, and précis) under the category name abridgment. Here is Fernald's coverage of the three terms of special interest:
An outline or synopsis is a kind of sketch closely following the plan [of a book]. An abstract or digest is an independent statement of what the book contains. ... A summary is the most condensed statement of results or conclusions.
This treatment of abstract, summary, and synopsis is identical to the treatment that appears in Fernald's English Synonyms and Antonyms, thirty-first edition (1914). In my view, while the descriptions of abstract and synopsis may still be accurate, the description of summary treats that word far more restrictively than most people do who use it today.
S. I. Hayakawa, Choose the Right Word: A Modern Guide to Synonyms (1968) groups abstract and synopsis (as well as abridgment, digest, outline, and précis) under the category name summary. Here are the relevant portions of Hayakawa's coverage:
These words refer to a short description of the main points of a longer work or presentation. Summary is the most general of these words, referring to any attempt to condense into as few words as possible an extended train of thought: a day-to-day summary of the proceedings in the murder trial; concluding each chapter with a summary of its main arguments. The word implies a pithy paraphrase, with no attempt to catch the style of the original. Also, the word almost exclusively refers to something that follows after and is based on the extended presentation, or even concludes it—as suggested by the common phrase in speechmaking: in summary. Abstract and précis both refer to summaries written most often by someone other than the original author; hence they are seldom part of the original presentation, though they follow it and are based on it. Like summary, they stress brevity and the schematic representation of essential points with no attempt to preserve flavor. Abstract most specifically refers to a scholarly or legal citation that gives the gist of what may be a complex argument or study: a quarterly containing abstracts of doctoral dissertations in progress; an abstract of the proposed legislation. ...
Outline and synopsis relate to précis in that they both retain the point-by-point ordering of the original; they are both most often a skeletal setting down of these points, but may be drawn up either by the author or someone else before, as well as after, the writing of the original. Within these possibilities, outline covers a wider range than synopsis. ... Synopsis usually refers to a plot summary of a piece of fiction. Ordinary prose sentences are most often used, rather than the numbered and lettered list suggested by outline. It may tell in capsule form events treated in a completed work or those planned for a projected work: submitting the first chapter of his novel and a synopsis of the unwritten remainder; writing synopses of novels submitted as candidates for film treatment. The word may also refer to a paragraph that retells previous action and introduces an installment of a serialized work of fiction.
Hayakawa's assessment of how people use summary is seems generally valid for today's usage, although in my experience an "executive summary" physically precedes a full report or article rather than following it.
Merriam-Webster's Dictionary of Synonyms (1968/1984) omits coverage of both overview and summary, but addresses abstract and synopsis as part of a larger group of words that also includes epitome, brief, and conspectus, under the category name abridgment:
abridgment, abstract, epitome, brief, synopsis, conspectus mean a condensation of a larger work or treatment, usually one already in circulation. ... Abstract implies condensation of a lengthy treatise or of a proposed lengthy treatment and stresses concentration of substance {abstracts of state papers} {an abstract of a lecture} ... Both abstract and epitome are used also in extended senses in reference to persons or things, the former stressing one or other (of the persons or things referred to) as a summary, the latter as a type representing a whole {a man who is the abstract of all faults that all men follow—Shak.} ... Synopsis and conspectus imply the giving of the salient points of a treatise or subject so that it may be quickly comprehended. Synopsis, however, often suggests an outline or coherent series of headings and conspectus a coherent account that gives a bird's-eye view {provide in advance a synopsis of the lectures}
As for overview, its meaning in modern English seems to be very close to that of summary, to judge from its brief entry in Merriam-Webster's Eleventh Collegiate Dictionary (2003):
overview n (1588) : a general survey : SUMMARY
