All medievalists love cartularies. What's a cartulary, you ask, and why are they so lovable?
A cartulary at its most basic is a book ("codex") into which are copied many charters (a charter is a carta in Latin, as in Magna Carta). When an institution (most commonly a monastery) had acquired a number of charters, it became clear that a big pile was unwieldy, to say nothing of difficult to save in case of fire or attack, when it would be easiest just to snatch up a book and run.
This was, remember, an era without a printing press, without copy machines, certainly without digital files. An original document was the only version in existence unless it was manually copied.
Although monks had been organizing the documents in their archives from the time they first started acquiring them in any numbers, the golden age of cartularies was the twelfth and thirteenth centuries. A monk or monks would carefully sort through and organize what they had on hand and then copy the documents into a book. The book, like the original charters, would be written on parchment.
Although to the modern eye the most sensible organization would be chronological, this was actually the least common way to organize medieval cartularies. Rather, the scribes organized topically. Often (although not always) he would begin with grants of privilege or immunity from popes, kings, and bishops. After that the material was most commonly arranged geographically, so that a charter from the emperor Charles the Bald (ninth century) confirming a monastery's possessions in a particular area would be found next to a very recent one donating a piece of a field. Every scribe organized the material slightly differently; there was no universal standard.
By far the majority of charters dating from before the twelfth century are now found only in cartularies, not as originals. Thus we would know substantially less about the early Middle Ages if it were not for these volumes.
Once a charter was copied, the original loose piece of parchment became much less important, which could lead to loss. Sometimes the original was on papyrus, disintegrating badly by the time it was copied. Where comparisons can be made, it is clear that most cartulary scribes were very conscientious, carefully copying what was before them. The first few dozen folios of a cartulary are always done very carefully and fully. At a certain point, however, enthusiasm might begin to flag, and the scribe would begin to abbreviate. But out-and-out forgery was extremely rare (unless, of course, the document the scribe was conscientiously copying was itself a forgery).
So why do medievalists like cartularies so much? It is because here is a record of important transactions at the monastery, revealing much about secular society as well as the religious house--most charters record the interaction of laymen and churchmen--and also revealing what the monks wanted to have remembered, and how. Social historians and historians of religion all use cartularies extensively.
For example, a charter reveals much about family structure if the donor lists all his relatives for whom he wants the monks to pray. It suggests power relationships if the donor's lord is expected to come and agree to a gift. Sales of property to the monks, land leased or exchanged, all suggest the complexities of the medieval economy.
And in reading through an actual twelfth-century cartulary, the modern medievalist can better understand the scribe eight or nine centuries in the past, whose handwriting becomes sloppy at the end of the day, who discovers he hasn't left enough room and has to squeeze the last few words in, who draws a finger in the margin pointing at an especially important transaction.