Note: Think of these being published on a continuum of time...

PRIMARY resources provide access to articles describing observational studies or experimental studies which:

• Are written by the actual researchers
• Describe only a single study, not a group of studies
• Are usually published in academic, peer-reviewed journals

SECONDARY resources provide access to synopses, structured abstracts, systematic reviews and meta-analyses.

1. Create summaries of information found in primary sources
2. Include reviews, systematic reviews, and meta-analyses

TERTIARY resources do not develop material themselves.

1.  Include reference/background/presearch/exploratory resources that broadly synthesize the literature and accepted practice 
2. Are not updated as often as primary and secondary literature
3. Example: Martindale: The Complete Drug Reference (2009) 

(With thanks to the people who created other library guides which I used to create this one)