As a core component of drafting your clinical questions and PICOs, as well as understanding how to approach your eventual search for evidence and information, it is important to understand the type of question you are asking. There are a variety of "Question Types" that apply to the EBM process. Below is a very brief overview of what those question types are, what they represent, and what types of evidence you may look for when asking particular types of questions.
Source: WCMC Medical Library - http://med.cornell.libguides.com/content.php?pid=295507&sid=5600835
As you search through databases and information resources, you will find evidence of varying degrees of quality and reliability. While you will need to critically assess your needs in every situation, the pyramid below highlights the general hierarchy of the various levels of evidence.
In addition to the chart below, the Oxford Centre for Evidence-based Medicine has an excellent page on how certain types of evidence relate to certain types of questions.