There are many different versions of the evidence pyramid but the main thing it does is demonstrates the evolution of the literature. An idea conceived in a lab turns into therapies and diagnostic tools tested with laboratory models, then in animals and finally in humans. The human testing begins with volunteers and goes through several phases of clinical trials before the therapy or tool can be authorised for use within the general population.
Randomised controlled trials are done next to further test the effectiveness and efficacy of a drug or therapy.
The amount of literature decreases as you move up the pyramid but increases in its relevance to the clinical setting. As you move up the pyramid, the study design is more rigorous and allows for less bias or systematic error.
Filtered resources appraise the quality of studies and may make recommendations for practice.
Evidence Based Practice (EBP) describes the practice of making decisions about patients and treatments based upon up to date and reliable evidence. For healthcare practitioners, this consists of "producing evidence; making evidence available; [and] using evidence" (Gray 2001, p14).
The following definitions come from the Centre for Evidence Based Medicine.
Sources for randomised control trials, cohort studies, case-control study, etc.