Publishers define relationships between their own content items (e.g., World Wide Web pages) and content items created or maintained by other publishers. A central namespace manager provides grouping labels that publishers insert into their content items. Content items having matching grouping labels are identified as related. Different classes of grouping labels can be used to define different types of relationships, and multiple classes of grouping labels can co-exist. Grouping labels are made available to an information server such as a search server, which uses the labels in identifying content items related to a currently displayed content item.