Unlike a library, the web is a relatively unbounded resource, to which there is no complete catalogue. With a library, it is usual to have a record of all books within it, and to be able to search those records with some guarantee that your search is comprehensive within the collection.

However, with the web we have a collection which is so vast and so changeable that even if we could catalogue it all, it would make little sense to do so. In response to this, we have to develop other techniques to find the information we want.