Different databases search different sources and types of information. Select a database by thinking about the type of information you'll need for your research.
Some databases contain the full-text of articles (for example, JSTOR), others are indexes and contain citations and/or abstracts, with links to the full-text (for example, Academic Search Complete).
Be sure you know the difference between the database name and the database provider (which is the brand or company) that hosts the database. For example, EBSCOhost is the name of the database provider that hosts Academic Search Complete but it also provides access to many other databases.