Essesntially, a
thesaurus organizing
words and
phrases, intended to cover all the topics that
library holdings (
books,
periodicals,
recordings, etc.) might be
classified under. The LCSH was developed by the
Library of Congress. It is a
controlled vocabulary originally designed for use by the LoC for
subject indexing in their
catalog, but has been used by many other libraries and even some online
bibliogrphies. Because libraries that use the
Library of Congress Classification System to organize their books do subject indexing according to LCSH, when you do a subject search in those libraries, you will have the best luck if you search on words/phrases that are entries in the LCSH.
This is the main Library of Congress site that deals with LCSH:
http://lcweb.loc.gov/cds/lcsh.html