Subject terms, also often referred to a subject headings or controlled vocabulary, are standardized words or phrases assigned to articles by indexers.
Subject terms limit the need to think of all potential synonyms, since a standard term will be applied regardless of which terms the author used.
Subject terms will only be applied to an article if that concept is a major theme of the article. Keywords for a term can pull in articles that mention a word, but either are using it in a different way or are just making a side reference to it.
For the same reason subject terms can limit to the most relevant results, articles that discuss a concept, but do not have it as the focus may not receive a subject term for that concept. In addition, subject terms are only as good as the work put in to apply them. Many databases are very meticulously indexed, but there can sometimes be gaps where articles have not been indexed with subject terms or a subject term is indexed inconsistently for any number of reasons.
In most cases, subject terms will be specific to a single database (there are exceptions such as MeSH being used in multiple medical databases). This means that you can't necessarily use the same subject term from one database when searching elsewhere.
Most databases will have an index of subject terms that can be browsed. From there you can usually directly add terms or sets of terms to your search (subject headings are usually arranged in a hierarchy where you can search either a specific term or that term and all sub-terms). Below are some examples of where to browse in different database platforms:
EBSCOhost
** For MeSH searches in Medline:
ProQuest
In the advanced search interface of most databases, you can select to search within an individual field (author, title, abstract, etc.). You will usually be able to select a field designated for subject terms/headings and search for keyword results in only that portion of the record.
Searching for keywords in the subject term field is not generally as comprehensive as browsing for the terms (especially since browsing will often include cross-references to relevant terms), but can provide a quick way to focus a broad search.
Even if you don't start with subject term searching, you can use the subject terms assigned to an relevant article that you find. The subject terms will almost always be hyperlinked, so you can click on a relevant subject term from one article to find all the articles in the database that were indexed with that term.