In the form of:

• Dictionary definitions of specialized words used in the field
• Short but authoritative Encyclopedia and Handbook Essays

What they're useful for:

• Basic, authoritative facts about a topic
• Brainstorming or refining a paper topic

This type of source in the Social Sciences is known as a

• Tertiary Source

In the form of:

• Articles from Peer-Reviewed Journals
• 'Gray Literature' - Scholarly works not published in peer-reviewed journals (Dissertations, Reports, Working Papers, Conference Proceedings)
• Scholarly Books (aka 'Monographs')

What they're useful for:

• Reports of primary research being done in a discipline
• Other researchers' reactions towards published research reports
• Opinion pieces discussing current developments in that field
• Secondary articles which profile the development and progress of specific lines of inquiry

This type of source in Biology / Life Science is known as usually a

• Primary Source

But some research articles (such as Review articles) are actually a

• Secondary Source

These databases are particularly helpful in sociology research: