A collection is a group of resources that have been selected for some purpose. Similar terms are set (mathematics), aggregation (data modeling), dataset (science and business), and corpus (linguistics and literary analysis).

We prefer collection because it has fewer specialized meanings. Collection is typically used to describe personal sets of physical resources (my stamp or record album collection) as well as digital ones (my collection of digital music). We distinguish law libraries from software libraries, knowledge management systems from data warehouses, and personal stamp collections from coin collections primarily because they contain different kinds of resources. Similarly, we distinguish document collections by resource type, contrasting narrative document types like novels and biographies with transactional ones like catalogs and invoices, with hybrid forms like textbooks and encyclopedias in between.

A collection can contain identifiers for resources along with or instead of the resources themselves, which enables a resource to be part of more than one collection, like songs in playlists.

A collection itself is also a resource. Like other resources, a collection can have description resources associated with it. An index is a description resource that contains information about the locations and frequencies of terms in a document collection to enable it to be searched efficiently.

Because collections are an important and frequently used kind of resource, it is important to distinguish them as a separate concept. In particular, the concept of collection has deep roots in libraries, museums and other institutions that select, assemble, arrange, and maintain resources. Organizing Systems in these domains can often be described as collections of collections that are variously organized according to resource type, author, creator, or collector of the resources in the collection, or any number of other principles or properties. In business contexts, the use of “collection” to describe a set of resources is much less common, but businesses organize many types of resources, including their employees, suppliers, customers, products, and the tangible and intangible assets used to create the products and run the business. Indeed, a business itself can sometimes be abstractly described as a collection of resources, especially when the resources are software components or services. (See endnote46[Com].)

A type of resource and its conventional Organizing System are often the focal point of a discipline. Category labels such as library, museum, zoo, and data repository have core meanings and many associated experiences and practices. Specialized concepts and vocabularies often evolve to describe these. The richness that follows from this complex social and cultural construction makes it difficult to define category boundaries precisely.

Libraries can be defined as institutions that “select, collect, organize, conserve, preserve, and provide access to information on behalf of a community of users.” Many Organizing Systems are described as libraries, although they differ from traditional libraries in important respects. (See the sidebar, What Is a Library?)

We can always create new categories by stretching the conventional definitions of “library” or other familiar Organizing Systems and adding modifiers, as when Flickr is described as a web-based photo-sharing library. But whenever we define an Organizing System with respect to a familiar category, the typical or mainstream instances and characteristics of that category that are deeply embedded in language and culture are reinforced, and those that are atypical are marginalized. In the Flickr case, this means we suggest features that are not there (like authoritative classification) or omit the features that are distinctive (like tagging by users).

More generally, a categorical view of Organizing Systems makes it matter greatly which category is used to anchor definitions or comparisons. The Google Books project makes out-of-print and scholarly works vastly more accessible, but when Google co-founder Sergei Brin described it as “a library to last forever” it upset many people with a more traditional sense of what the library category implies. We can readily identify design choices in Google Books that are more characteristic of the Organizing Systems in business domains, and the project might have been perceived more favorably had it been described as an online bookstore that offered many beneficial services for free.