Digital libraries provide reference lists, citations, and metrics for evaluating impact and influence. A reference list contains sources used for research organized by type like books, journals, papers and theses. Citations occur within the text and identify author and source for direct quotes and paraphrases. They provide additional publication information. Citation networks, documents with high impacts, or similar patterns can be identified. The i10-Index represents the number of publications with 10 or more citations. Charles Darwin's i10-Index is provided as an example. The H-Index reflects the number of papers cited at least as many times as the index number, measuring both quantity and impact with a single value.