A hanging indent is the indentation of all lines other than the first within a paragraph. These are typically only used in bibliographies, as they allow for easy differentiation between sources.
APA, MLA, and CMOS all require the use of hanging indents in reference lists.
For information on hanging indents in the Publication Manual of the American Psychological Association see section 2.12, "Reference List."
For information on hanging indents in The MLA Handbook see section 1.6, "Placement of the List of Works Cited."
For information on hanging indents in The Chicago Manual of Style see sections 1.63, "Bibliography or Reference List;" 2.11, "Tabs Versus Indents;" and 2.24, "Format for Bibliographies and Reference Lists."