Databases: The term "databases" (as used in libraries) refers to digital collections of information. Databases contain primarily journal articles, but many include images, data, clinical trial results, patents, theses and much more. Some return full text articles; others supply the citation and links to full text. Many have a specific focus, while others have a broad scope.
Citation: A citation is a succinct summary of an item, with information provided in a standardized way. A citation typically consists of:
Example: Alosco, M. L. (2012). Cognitive function and treatment adherence in older adults with heart failure. Psychosomatic Medicine, 74(9), 965-973.
Why do I need to know about citations? Databases often supply only the citation - not the full text. This allows the database to respond more quickly and still provide enough information for the item to be located.
Abstracts: An abstract is a summary of an article, usually no longer than a paragraph or two.
Why do I need to know about abstracts? Often when searching on a keyword you can select the fields in which the keyword appears. For example, do you want articles that have that keyword in the title only, or is anywhere in the text ok? A good search strategy is to see if your keyword shows up in the abstract. That way you know it's an important concept in the article, even if it didn't make it into the title.
Literature search: not a book review, but a non-biased summary of the literature available on a specific topic.