LC Subject Headings are used to describe the materials in a library collection so they can be found in a systematic manner. LCSHs let us use a "controlled vocabulary" to search for the information in a logical manner.

“Much more problematic than the description of documents is the description of the content, meaning, purpose, and related features of the messages (the representations of knowledge) found in documents, document collections and parts of documents. Some type of subject indexing, subject cataloguing, or classification is required in order to organize and provide access to messages in documents whenever particular documents are not known in advance.

“After documents and their content have been described, these descriptions must be stated in terms that will match the query terms of searchers. This requires some sort of vocabulary control or management. The traditional approach has been to require the use of a restricted set of terms, with the occasional provision of cross-references from alternative terms.”
— Ray Prytherch, comp. Harrod’s Librarians’ Glossary. 8th ed. 1995, “Organization of Knowledge.”
 

LC Subject Headings are:

• A formal, consistent vocabulary system used by most libraries to describe the subjects of books and other materials.

• Consist of designated words or groups of words under which all materials on that topic can be found.

• Of many possible synonyms that could be used to describe the subject of a book (e.g. garbage, trash, waste, refuse), only one term or phrase is selected as the formal subject heading.

Understanding the difference between the LC Classification and LC Subject Headings can be confusing.  LC Classification and LC Subject Headings are similar - they're both related to the topic of the book. LC Classification and LC Subject Headings are related to one another. For example, the LC Classification number E185 corresponds to the LC Subject Heading "Afro-Americans."

They are not the same, however...

The table below compares some of the differences between the Library of Congress Classification System and Library of Congress Subject Headings.