Citations may be made by the examiner or the applicant/inventor. They comprise a list of references that are believed to be relevant prior art and which may have contributed to the "narrowing" of the original application. The examiner can also cite references from technical journals, textbooks, handbooks and sources. The citations practices of the EPO differ substantially from the USPTO. Applicants to USPTO are legally required to include a full list of prior art known or believed to be relevant (“duty of candor”). At EPO, no such requirement exists for applicants.
There are 2 types of citations: forward and backward citations. Suppose patent B is referring to patent A in its description, then A is a backward citation for B and B is a forward citation for A.