What are tags?
Tags are a simple way to categorize books according to how you think of them, not how some library official does. Anything can be a tag—just type words or phrases, separated by commas. Thus one person will tag the The DaVinci Code "novels" while another tags it "trashy, religion, mary," and still another only "summer home." Tags are particularly useful for searching and sorting—when you need a list of all your novels or all the books at the summer home.
Once you have a hundred books or so, you need some way to organize them. Library subject classifications, including that of the Library of Congress, are one solution. For most personal libraries, however, they aren't much use. "Tags," informal, personal markers used on blogs and sites like Flickr and Del.icio.us, provide a better model.
Here are two examples from my (Tim's) experience:
Tags can also mark "favorites" or "books to read." I've used the tag ben's to mark books I should return to my friend Ben. (That I included them in my catalog is, however, a bad sign for that!)
In addition to being a way to organize your own collection, social content sites like Flickr and Del.icio.us have shown that large numbers of different users' tags produce categorization structures ("folksonomies") that can surpass traditional taxonomies. (See Clay Shirky's talk "Ontology is Overrated: Categories, Links and Tags" for a stimulating discussion.) As LibraryThing grows, I expect to use this data in new and interesting ways.
What are works?
The purpose of works is social. Books that a library catalog considers distinct can nevertheless be a single LibraryThing "work." A work brings together all different copies of a book, regardless of edition, title variation, or language. This works system will provide improved shared cataloging, recommendations and more. For example, if you wanted to discuss M. I. Findley's The Ancient Economy, you wouldn't really care whether someone else had the US or the British edition, the first edition or the second.
What is a book then?
The "This Book" data on the book information page is particular to YOUR copy—the distinct edition that's sitting on your bookshelf. None of the combining and separating of works will change anyone's personal book data.
What works should be combined?
What works shouldn't be combined?
What are some edge cases?
Remember, the purpose of the system is social. Therefore, I feel that some edition or language differences are so major as to be socially significant. Two examples: