Approximately "the obligatory, the binding", in the general sense the name for the collective tradition of Biblical textual commentary and Hebrew grammatical systematisation which reached its most productive period in the early middle ages. The most important work of the Masorite scholars was the fixed vocalisation of the consonant-text of the Hebrew Bible. Narrowly defined, the Masora is the collected text which resulted from this work.
The first-century Jewish historian Josephus tells us that in his time, some centuries after Hebrew had died as a living language, the Sopherim, or scribes, no longer dared to edit or correct the text of the Old Testament; in effect, the text of his time was the sacred, unchangeable basis of scripture. But there were two problems. Firstly, Hebrew used an originally consonant-only script, in which only long vowels, by employing semi-vocalic consonants, can be indicated. Secondly, since Hebrew had long since been replaced by Aramaic as a popular language in Palestine, the exact pronunciation of the texts could be preserved only by oral transmission, and as time went on and the Jewish people more scattered, this became more and more difficult.
The first attempts to provide a solution were awkward transcriptions into Latin or Greek characters, as preserved in the Hexapla of Origen. This never systematically carried out, and was replaced in the 5th century by the precursor of the well-known Hebrew vocalic indicators, modelled on the Syriac script used by the Nestorian Christians. The eastern and western Masorites of the 7th and early 8th centuries, in Babylonia and Palestine, created two imperfect systems in which the semi-vocalic letters waw, yod, aleph, ayin, and he were simplified and written above the consonants to indicate the following vowels; the readings could thus be determined without literally changing the letters of the current text.
Around the year 760, Anan ben David founded the Karite movement, whose basic principle was the challenge of Talmudic traditions in favour of literal, biblical interpretations. The natural result was a renewed interest in textual criticism and exact determination of pronunciation, and under this impetus the Masorites, too, flourished. The main burden of work was carried out in the Palestinian city of Tiberias under the leadership of five generations of the family of Ben Asher. It was this Masorite circle which developed the Palestinian system into the precise but flexible Tiberian, still used (with a few improvements) today. Pronunciation (including, under the influence of Syriac, the double pronunciation of the consonants BGDKPT) and grammar (under heavy influence of the Arabic grammarians) were systematised - a mammoth achievement of organisation for a language in chaos. Statistics of the number of words and letters in each book of the Bible ensured that copyists could proof-read their work accurately. Perhaps most importantly, though, and despite the great gap of time since Hebrew had died out, this work was an attempt to recreate the original text, rather than provide a new interpretation - a concept which European philology wouldn't really discover until the humanists.
Despite similar efforts (most notably by the circles of Pseudo-Ben Naftali), the system was quickly accepted as a standard, in no small part thanks to the philosopher Moses Maimonides' declaration of its supremacy at the end of the 12th century. It is worth noting that all Bible-texts which were written after this time display very few discrepancies or alterations.
The text of the Masora in the narrow sense of the term consists of three parts, written mostly in Aramaic, with some Hebrew as well. The first is the Masora Parva, the little Masora, containing margin-notes to the explanation of the biblical text. It includes the markation of passages providing text-critical or dogmatic problems for the Masorites, the order of words or verses, suggestions for improvements on unexpected forms (Sebirim), and variant readings for the written text (Ketib/Qere, or "what is written" and "what is read"). The second is the Masora Magna, the greater Masora, containing mostly explications of the Masora Parva. Thus, where the latter may add the note that the phrase "In the beginning..." of Genesis 1:1 occurs five times in the Bible at sentence beginning, the former will give the list of passages where this occurs. The third part is the Masora Finalis, the End-Masora, an alphabetical index of the passages and commentaries of the first two.
hmmm...I've culled most of this from random notes and from E. Würthwein. Der Text des Alten Testaments, 3. Auflage. (Stuttgart 1966). While a worthwhile book, I'm guessing most will find it quite useless, mainly since it's in German. I'll try to find a better reference in English later.
Also, this is seriously out of my field of expertise, but nobody else seems to want to write this. Feel free to /msg me on gross or minor errors on my part in this write-up.