Subject headings are very specific to a particular set of databases, or library catalogues. They are part of a controlled vocabulary, where a term is only used in a subject heading when the item is specifically about that term. Usually a person has assigned these terms based on their knowledge of what's in the book. For example, books with "Marriage" in their subject heading are mainly and specifically on the topic of Marriage.
Keywords are simply words that can appear anywhere in the title, subject heading, abstract or even full text of the article. If you search Marriage as a keyword, then you can locate books that mention marriage on a single page, or have a short chapter addressing Marriage.
Subject headings can narrow your search if your results are too broad. You can also combine keywords and subject headings in the advanced search in any database to narrow or broaden your results.
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