Here's a good video that explains basic search strategy using keywords, subject headings, and Boolean operators.
When constructing a search strategy, you will first identify the key concepts from the research statement. Unlike natural queries (i.e. typing the entire sentence) that you probably use for web searching, in the database you will only need to put couple of major words. Keyword could be words in the title, journal, abstract or subject heading. It will pick up an article even if the the word is not the main focus of the article.
Unlike keywords, Subject Headings are the major content in the article. A search on the topic child abuse as a keyword may retrieve some articles with 'child,' some with 'abuse' and some with the phrase 'child abuse.' But if the subject heading is 'child abuse' then the article is focussed on that topic. Your search may be much broader retrieving more articles with keywords. Searching with subject headings, you are likely to get fewer but more precise results. Subject headings are also referred to as 'descriptors' in some databases.
Some databases have a thesaurus - a powerful collection of subject headings or standardized terms created by the indexers. Controlled vocabulary provides more effective searching capability in databases where a thesaurus is available. For example you may think of the word 'substance abuse,' but the database thesaurus indexes it as 'drug abuse.'
Use the thesaurus to find keywords and phrases and apply these search terms in the subject field (SU or DE). The results may be better for searches of thesaurus terms in the subject field than for a simple keyword search of the title or abstract of an article.
Make sure to consult the following thesauri when searching these databases:
MEDLINE- MeSH terms (Medical Subject Heading)
CINAHL - subject headings
PsycINFO - thesaurus from APA
Here's a good tutorial from Trinity Western University that explains the PsycINFO Thesaurus.