Keywords are a series of key words that help a researcher discover what information can be found in a particular article or book. Keyword searching is a broad form of searching that will enable you to discover if a specific term appears in an article, an abstract, or somewhere else on the page.
Subject headings are made up of controlled language or controlled vocabulary that is standardized across a database. Searching with subject headings may be described as a more refined form of searching that will enable you to discover sources that you know cover a particular subject.
Different databases have different subject headings, and some databases do not organize sources with subject headings at all. Using subject headings and keywords together is a common research practice.
Knowing the right keywords or subject headings to search will help you start your search off right. Often, when you aren't finding good information - or any information at all about a topic - it is because you haven't yet figured out the correct words to use in your search.
The best thing you can do before ever getting into a database is to think about the keywords that will best represent your topic. Write down your thesis statement and pull out the major terms in it. Then, think of as many different ways as you can to say those key terms. For example if you were researching:
Chronic disease among minority populations in urban areas.
Some keywords in this sentence are: "chronic disease", minority, and urban. However, searching for these terms will get you only a fraction of the material that is out there, because if a different keyword is used to express the same concept you need to search for that keyword as well. So, other keywords that would be useful to use when researching this thesis topic would be heart disease, stroke, cancer, diabetes, obesity, and arthritis (to represent chronic diseases); Asian American, Black, African American, Hispanic, Latino, Native Hawaiian, Other Pacific Islander, American Indian, Alaska Native (to represent minority populations); and other words to represent urban such as inner-city, metropolitan area, and city. You could also refine your search, and your thesis, by narrowing to a specific sub-set of a population (e.g., adolescents).
If you were conducting the search using Medical subject headings, which you will find in PubMed and other medical databases, it might include: chronic disease; minority groups and minority health; and urban populations and cities.