Cataloging which describes the particular work in hand, according to a particular cataloging code such as AACR2 (Anglo American Cataloging Rules, 2nd Edition), RAK (Regeln für Alfabetische Katalogisierung), or RDA (Resource Description and Access). [RDA is the working name for AACR3 and have not yet been ratified as a replacement for AACR2]
Descriptive cataloging describes what the information object is, not what it is about. "Aboutness" is the purview of subject cataloging and is not touched upon by descriptive cataloging. Descriptive cataloging sets out to provide access points. It requires the descriptive cataloger to identify the title proper of a work, the person responsible for the work (personal author, corporate author, etc.), and the extent of a work (physical dimensions or file size or URL web-address, etc.), as well as to determine the existence of uniform titles or alternative/parallel titles, and if the work belongs to a series or not, as spelled out by the particular descriptive cataloging code being used.
Seymour Lubetzky made an important distinction in this subject when he pointed out that the book is merely the material object and the "work" is the intellectual creation of the author. The object it is contained in may change format but the "work" remains the same.
Doing Descriptive cataloging with MARC21 also entails ensuring the fixed field elements are filled out correctly.