However, you can describe it in a paragraph: it's a handwritten book that's 230+ pages long, very probably about 500 years old, and filled with strange words and obscure pictures no-one can understand. I call it "a Scooby Doo mystery for grownups", but one where everyone is trying to pin the blame on a different janitor: and so the story loops endlessly, as if on a lost satellite cartoon channel.
For once, the Wikipedia Voynich Manuscript page falls well short of being genuinely useful: the VMs is so contested, so politicized, so intensely rubbish that the whole neutral tone Wiki-thing fails to please (I gently satirized this in my VQ questionnaire). Bucketfuls of worthlss opinions, and endless pussyfooting around: throw all that junk away, I say, and start from scratch. *sigh*
But if Wikipedia's faux-scientific neutrality can't get you started, what can? If (like me) you are a fan of Ambrose Bierce's "The Devil's Dictionary" (1911), your ideal introduction to the Voynich Manuscript might well be succinct, partial, and cynical (in fact, almost toxically so). In this vein, I heartily recommend "Folly Follows the Script", an article by Jacques Guy (AKA "Frogguy") in the Times Higher Education supplement from 2004. While ostensibly reviewing Kennedy and Churchill's recent book on the VMs, Guy rips apart a lot of the pretension and falsity that now surrounds the manuscript, in particular Gordon Rugg's much-vaunted (but actually resoundingly hollow) hoax papers. Which is, errrm, nice.
If you prefer lots and lots (and did I say lots?) of data, the best introductory site by miles is Rene Zandbergen's excellent voynich.nu, in particular his "short tour", and the even shorter tour. But frankly, it's hard for most people to care about Newbold, Petersen, Friedman, Strong, Brumbaugh, O'Neill, Feely, Manly, and even John "The Brig" Tiltman unless you've already lurched over the line into Voynich-obsessive mania: none of them could read a word of the VMs, and they're all long dead.
Alternatively, if you prefer a kind of gentle postmodern defeatism, I could happily recommend a very readable article by Lev Grossman called "When Words Fail", which first appeared in Lingua Franca magazine way back in April 1999: sadly, nothing much of substance has changed in the intervening decade (or, indeed, over several preceding decades too).
This might seem a horrible thing to say, given that so much ink has been spilled (and, more recently, so many HTML tags wasted) on the VMs over the last century in the honest pursuit of this wonderful (yet devastatingly cruel) enigma. But we still know next to nothing of any real use: the kind of intensely Warburgian art-historical research I've been slaving over for the last six years seems totally alien to most 'Voynichologists', a title that perpetually hovers too close to David Kahn's Baconian "enigmatologists" (see "The Codebreakers" (1967), pp.878-9), with their "deliriums, the hallucinations of a sick cryptology".
All of which is to say that both cynicism and nihilism are probably good starting points for reading up on the VMs: a century of careless credulity has got us all nowhere. But this is not to say that I am pessimistic about any advances being made. In fact, I would say that "the Devil's in the details" or the alternative "God is in the details" (both of which are sometimes attributed to Aby Warburg!) to flag that, beyond the superficial flurry of foolish and wishful opinions out there, I think there are things we can (and eventually will!) know about the Voynich Manuscript; but that for the moment these remain hidden in its vellum margins.
All of which is another story entirely...