A collection of Welsh tales first preserved in the Peniarth manuscripts (ca 1200?), then the White Book of Rhydderch (Llyfyr Gwyn Rhydderch) written about 1300-1325; and later in the Red Book of Hergest (Llyfyr Coch o Hergest), written between 1375-1400. Most scholars believe that all but the three "romances" date to the tenth or eleventh century, and are based on much early mythology.
The name is a misnomer, as the word "mabinogion" doesn't exist in Welsh. It was a mistake made by the scribe, existing only at the end of Pwyll pendeuc Dyfed. The real word is "mabinogi," which some translate as "tales for youth," "tales of the Mabon" and "tales of the hero." They derive this meaning from "mabon" or "meibon"--meaning a young man or youth. It is also the name of a god, Mabon ap Modron. This name only applies to the first four tales.
The tales can be divided into four sections (five if one includes the story of Taliesin):
The Four Branches of the Mabinogi: detailing the exploits of the Children of Llyr and the Children of Don; more specifically, it chronicles the life of the hero Pryderi ap Pwyll. This section is the most clearly mythological, with much material analogous to the ancient Irish sags:
The Histories: These two stories deal with early British history: they are not unlike episodes out of Geoffrey of Monmouth's History of the Kings of Britian:
The Tales of Llys Arthur: These are tales generally only found in Wales; they have no known Arthurian French analogues on the continent, though Culhwch ac Olwen is reminiscent of the story of Jason and the Argonauts.
The Franco-Cymric Romances: These have obviously been influenced by the French romances of Chretien de Troyes; still, they contain some native elements, and are not simply derived from Chretien, but influenced by:
When Lady Charlotte Guest translated the books, she added the story Hanes Taliesin; however, the manuscript from which that story comes is quite late in date, possibly the sixteenth or seventeenth centuries.
There have been four translations into English:
1839--1849: translated by Lady Charlotte Guest in seven volumes. This is the only version which features Taliesin. It now exists as a Dover edition, which is unfortunate, since they don't print her end notes, which are full of folklore not recorded elsewhere (at least not in any form available to the average 21st-century hobbiest). Otherwise, if you can find a copy of the 1902 J.M.Dent/Everyman edition, buy that. (I got mine in London for £5. I was ecstatic.) There is--published only in Britain--a new version with illustrations by Alan Lee, IIRC. However, it isn't published in America. To read the original dedication, click here.
1948: translated by Gwyn Jones and Thomas Jones. This is the most accurate translation--that is, word-for-word--but because of this it is somewhat unreadable. Published by Everyman.
1976: translated by Jeffrey Gantz. This is the most readable translation, written in modern English but still fairly true to the text. Published by Penguin.
1986: translated by Patrick K. Ford. This is now out-of-print. It doesn't feature the Romances, but does feature Taliesin. Published by University of Calfornia.
Aside from being one of the only real preservation of Welsh mythology (the other being The Welsh Triads, also found in the Red and White Books), the Mabinogion has influenced:
The Chronicles of Prydain by Lloyd Alexander;
The Dark Is Rising Sequence by Susan Cooper;
The Mabinogion Tetralogy by Evangeline Walton;
The Mists of Avalon by Marion Zimmer Bradley;
Gate of Ivory, Gate of Horn by Robert Holdstock;
The Owl Service by Alan Gardiner;
A Swiftly Tilting Planet by Madeline L'Engle;
Maiden Castle by John Cowper Powys;
The Chronicles of Narnia by C. S. Lewis
James Joyce mentions the book in Ulysses, in the same breath with the Upanishads.