But the concept is essential to understanding the ways that information is organized and managed in the digital word, and also valuable in keeping track of your own data. Or datums. In some ways, metadata functions as the new Dewey decimal system, the card catalog for the computer era, the index for the information age. Modern librarians talk about metadata more than microfiche; your computer makes metadata every moment, and uses it to find things later.

For example: if you were to save this article right now on your computer, the file would contain not only the data (the words you are reading) but information about the data, such as the date it was created, the date you saved it, the application that reads it, and its length in characters. When you open this file, you see its data, but its metadata is hiding.

Those educated in the classics will recall that the ancient Greek word meta appears in the first sentence of Plato's republic: κατέβην χθὲς εις Πειραια μετὰ Γλαύκωνος, I went down to Piraeus after Glaucon . Most scholars translate meta ( μετὰ ) to mean after or beyond. As in the word metaphysics: while physics (at least before Einstein) explains the here and now of concrete and tangible things and events, cosmology and religion deal with imaginary concepts well beyond the reach of commonplace rationality; they are thus after or beyond physics. Aristotle's chapter on these topics comes after his chapter on physics, and so was called μετὰ φυσικά, metaphysics.

Moving forward in history, we get the word data from the Latin, as the plural of datum, which is a past participle of the verb dare, to give. So data are collections of information that is given, taken at face value. What you see is what you get. The words you are reading are data; the date you save the file is part of the metadata for this document.

I have found metadata to be my friend. I have over 2000 images in the collection on my computer. Before metadata, when I went to find the picture I was looking for, it took forever, scrolling through pages and pages of little thumbnails. But once a friendly librarian showed me how to add metadata to my images as I collected them, I can find what I need quickly. And I know where it came from.

Let's suppose I am preparing a lecture on Napoleon (who thought he could build an empire bigger than Greek and Rome combined) and am looking for images to include in my slides. I have a photograph that I took in France on my digital camera, of a statue of the emperor himself riding on a horse. I import the photo into my computer. As I save it, I add some metadata about it: Napoleon on a horse, statue, Damnedifino, France, HI-456. The last item is the number of the course for which I am preparing the lecture. The image-management software I am using, iPhoto, automatically adds some additional metadata: the date the photo was taken, its filename, it's format (JPEG), and its size (640 kilobytes).

The picture of Napoleon is the data; it's what I see and what I will display in my slide show. All those other items are the metadata, information that describes the data itself. iPhoto, like most other good software, saves the metadata along with the data, and lets you use the metadata to find the picture later. So when I am searching next year for a picture of a horse to help teach my granddaughter a new vocabulary word, I'll be able to search my image collection for the word horse, and I'll get Napoleon. Along with any other horses I might have collected. In the same manner I can quickly find all the images destined for History 456, or all the pictures I took in France on October.

Librarians are doing the same for books, maps, recordings, objects and other things they are responsible for: they are adding metadata. They are tagging these items (they call them assets) with metadata. And organizing the metadata into online databases that are searchable and sharable, so as to facilitate inter-library research. And to help you find what you are looking for, or uncover items you did not know existed.

In our own way, we can copy the librarians. We can get into the habit of adding metadata to our collections of digital assets. As I overheard one librarian say to another, "You can shove that metadata right into your own assets." Here is an example of how it might work:

1. Locate with the help of Google Images a painting of Napoleon by his contemporary Jacques-Louis David.
2. Copy the image to the iPhoto library on my computer.
3. Copy the URL of the image from the National Gallery web page.
4. Paste the URL into the Info box on iPhoto.
5. Copy the Gallery's summary of this painting from the web page: The painting is an artful contrivance to convey three aspects of his public image: soldier, emperor, and administrator.
6. Paste the summary into the information box on iPhoto.
7. Type into the information box, HI-456, Napoleon in his study, David, 1812, hand in vest . (Yes, this is the famous painting that we all remember.)

What I have now is not only the image itself, but some valuable information about the painting that will help me later. The metadata will help me find it, understand it, cite it, and compare it. Adding metadata is a good habit, a discipline we should practice ourselves and teach to our students. I teach them to follow these steps as they conduct their online research:

1. Search. Use the tool that works best for you on the collections most relevant to your task.
2. Save. Copy the data to your own computer, not to a file in the Documents folder or on the desktop, but to a program that manages digital assets, like iPhoto or Picasa.
3. Cite. Copy the URL of the data (the image), and paste it into the information box for the item you just saved.
4. Signify. Give the item a title, and some words to remember it by. Not image45678.jpg, but Napoleon in his Study.
5. Specify. Add some more metadata: a description of the contents; the reason you gathered it; how you plan to use it; some unique adjectives. (The asset-management program will provide other metadata automatically, such as size, date, and format.)