As described in the Preface, the growth of scientific knowledge is predominately incremental - we build on past knowledge more often than we displace it. Thus, the first pillar of science - a communal collection of knowledge - requires mechanisms for preserving and disseminating knowledge within the scientific community. By far the most important mechanism in use today is the scientific publication. Although there are many forms of scientific publication, the two most common are the conference presentation (with or without some non-peer-reviewed written text) and the peer-reviewed journal paper (both in print and online).
Because virtually all scientific advances build on past knowledge, it is critical that the new work be placed in the proper context with respect to the past work upon which it builds. The primary mechanism for this is the citation (or reference). Within a scientific paper, references are placed to other works, creating points of contact with the communal collection of scientific literature in order to fit the new work into the web of knowledge. But given the skeptical attitude that is also a part of science, citations are also used to help readers verify the quality of the new work and assess the strength of its conclusions.