Books, journals, encyclopedia, standards, statistics, laws, patent, print and electronic, databases, online resources, experts... There are many information sources.
Knowing the essential information sources in your field is a great help when searching information. Disciplines differ in publication cultures and practises. So-called "hard sciences" focus on international refereed journals, while national books and journals are more important for humanities and social sciences.
Choosing and finding the appropriate sources for your need is crucial, as well as learning to use the key databases in your field. However, bear in mind that it is sometimes possible to find relevant information in sources which are not directly related to your field!
Full text databases include articles in full text, as well as references
Bibliographic databases include references to where the article has been published, and possibly an abstract. The article is not necessary included in full text, which means that you have to search it elsewhere. The article might be available in another database that your university subscribes to.
Citation databases include citations between articles. Who has cited the article and how many times has it been cited in all?
Note! Most databases are hybrids between full text and bibliographic databases. Some journals/articles in the database are available in fulltext but not all.