Primary scientific literature presents original research and contains certain hallmarks such as:
Examples of primary scientific literature are: case studies/reports, clinical trials, observational studies, randomized controlled trials, some theses/dissertations.
Secondary scientific literature synopsizes or summarizes the current state of affairs on a certain subject.
Examples of secondary scientific literature are: review articles which review published materials, textbooks, and reference books.
Conference proceedings can be either primary or secondary depending on the conference. An example of a secondary conference paper is:
Byers, C. G. (2014). Management of Diabetic Ketoacidosis. Paper presented at the American Board of Veterinary Practitioners Symposium.
This paper details the veterinarian's own experiences with treatment of this condition, and does not contain any original research.
An example of a primary conference paper is:
Lopes, R., Suchodolski, J., Steiner, J., Hill, S., & Lidbury, J. (2015). Serum Markers of Vitamin B Metabolism in Cats with Hepatic Lipidosis. Paper presented at the American College of Veterinary Internal Medicine Forum.
This paper details an experiment undertaken on a group of cats. This is an example of original research.
For a more in-depth treatment of the types of research pertinent to the field of veterinary medicine, see the article: Understanding and Evaluating Veterinary Clinical Research by Buhles et al.
Evidence-Based Veterinary Medicine - the formal strategy to integrate the best critically-designed and statistically evaluated research available combined with clinical expertise as well as the unique needs or wishes of each client in clinical practice (Evidence-Based Veterinary Medicine Association)
The top tier of evidence-based medicine is represented by Systematic Reviews and Meta-Analyses.
Systematic Reviews - a review of a clearly formulated question that uses systematic and explicit methods to identify, select, and critically appraise relevant research and to collect and analyze data from the studies that are included in the review. (http://us.cochrane.org/frequently-asked-questions)
Meta-Analyses - a study designed to systematically assess previous research studies to derive conclusions about that body of research. Outcomes from a meta-analysis may include a more precise estimate of the effect of treatment or risk factor for disease, or other outcomes, than any individual study contributing to the pooled analysis. (Haidich, A. B. (2010). Meta-Analysis in Medical Research. Hippokratia, 14(Suppl 1), 29-37.)
More information on evidence-based veterinary medicine can be found at the EBVM Network site.