In developing searching strategies, keywords are the terms that are used in an article or abstract. Many times, keywords will be all you need. However, if you don't guess the term the author used for your topic, you will not find that article.
In indexed databases, no matter what term(s) an author uses (or neglects to use), there will be one subject heading applied for that topic. In the following example, you would have to search with every keyword term in order to find all the articles on cat scans. However, in PubMed, you only need to use one subject term to collect all indexed articles on cat scans.
When selecting keywords, consider all the different options an author might use for that concept, then link them together using the Boolean OR term. AND together different concepts.
The USC Libraries' Developing Keywords tutorial can help you practice identifying potential keywords to search a database.
Always use keywords to search Non-Indexed databases.
Indexed / Thesaurus / Controlled vocabulary
When searching with the controlled vocabulary, choose the broadest term (higher on the hierarchy) that applies to your topic. You may also want to OR a related keyword. AND together different subjects for your search.
For more details, visit: Indexed Databases.
PubMed MeSH Hierarchy Tree
Example: (Climate processes [mesh] OR "climate change") AND california