Style of handwritten codex typical of the 4th-16th centuries, marked by ornate capital letters to mark sections, the use of color (including gold leaf) and interior illustration, and the succession of hands and colors
While illustration and the use of ornamented letters was not unknown to Latin scribes, the Roman callegraphic ideal was one of uniformity throughout: even breaks betweeen words were considered unsightly. The scroll, as opposed to the codex, form meant that illustrations were subject to chip off the page in the course of normal wear and tear. However, in the latter days of the Empire, we began to see folded and/or flat leaves and a deviance from the Roman Capital hand in a form termed Uncial.
The Uncial hand was invented, not by scribes, but by sophisticated Alexandrians of the 3rd century who often had to write letters and other documents in both Greek and Latin. Somehow, it became fashionable to write Latin with Greek-looking letters (but not, apparently, the opposite) to produce a quickly written hand suitable for chic-looking party invitations and the like. Nowadays, they'd just keep a fancy font on their computers, but the upshot was that some people started writing like this all the time, especially for documents near and dear to their hearts, such as their personal copies of the Gospels or Psalms, which often got marked up with underlines, doodles on the margin, personal comments and the like. As you might imagine, this was frowned upon by both Pagan (who thought Capitalis Monumentalis and Rustica quite enough, thank you very much) and Christian (who thought the notion of writing the message of humility and discipline in the equivalent of girly handwriting repugnant) scribes. However, as things got tight in the Eastern Empire in the 4th-6th centuries, and refugees began to sell the books on their nightstands to missionaries bound for Gaul, it was, well any port in a storm. Soon most of the newly literate scribes working in Gaul and other points West were learning to write from these books, and producing codices of their own.
There, the matter would have stood, except that books of any kind were rapidly becoming scarce, literacy was declining, and the price of parchment was becoming exceedingly high. (For those whose notions of the Middle Ages comes from playing AD&D, the current market price of animal parchment is $25 USD/sq.ft -- and this is unimaginably cheap, by medieval standards.) Books, especially Gospels, were becoming a gift literally fit for a king, who might not be able to read, but knew the value of ivory, gold, and purple ink. Consequently what books were copied became more ornamental and impressive-looking than legible. This was marked by ornamented capitals, interior illustration, and the use of pagan "knotwork" (as in the book of Kells but widespread elsewhere as well), and a growing freedom in letterforms, the use of separate capital and small letters, and extravagantly joined letters, a trend that reaches its zenith (or nadir) in the Luxeuil Script of the 7th century.
With the accession of Charlemagne, however, a new drive for literacy eminated from the Palatine school headed by Alcuin, who instituted language reforms around the year 800 which purged handwriting of its excesses, and restored what he thought was the classic, austere, Roman hand: Egyptian uncial! Henceforth, letterforms became cleaner and clearer, illustrations became separated from the text (though still an integral part of the page) and more numerous.
With the eleventh and twelfth centuries came two other developments that were to have a great influence on the illuminated manuscript as we know it: the rise of universities and of Nominalism. Without going too heavily into the minutiae of Scholastic philosophy, it suffices to say that with the rise of a secular, as opposed to a purely monastic or cathedral-based, tradition of teaching, more books were needed than ever before, both copies of old ones and of the many newer ones that began to excite interest. With the rise of Nominalism, there arose an interest in realistic, as opposed to stylized, art. Finally, many of the laity were becoming literate, thus increasing the demand for such books as romances, chronicles, and Books of Hours all of which were often extravagantly illustrated. Les tres riches heures, produced by the Limburg Brothers, is typical of this period: unlike the Book of Kells, we see gold being used as a highlight and to model figures, as opposed to its use in large flat planes, realistic figures engaged in secular activities in landscapes, instead of against figured or monochrome fields, and the liberal use of ultramarine, a pigment derived from lapis lazuli.
With the invention of movable-type printing and the introduction of paper into the West around 1450, the illuminated manuscript's days were numbered. Nonetheless, many people far into the Renaissance preferred hand-written books to printed, and there are some classically-styled illuminated books to be found in this period, as well as books in which the bulk of the text was printed and capital letters, pictures, and so on applied by hand -- the Gutenburg Bible is of this type -- as well as numerous books that simulated illumination by printing with colored inks. Despite the clear victory of type, however, illuminated books have been produced, albeit on a limited basis, down to the present day.