A useful way of thinking about source material is discussed on the How to Use Sources Effectively in Expert Writing page. Much of the information that follows is from that website.
Use the I-BEAM method to figure out what work you mean each source to do in your piece of writing. I-BEAM stands for Instancing, Background, Exhibit, Argument, & Method.
Instancing is the use of sources to indicate the context and nature of the question you are addressing, or even its very existence.
These sources will probably show up in your introduction, to help define your project in light of what has come before and establish a context in which your reader can see the importance of your project.
Background source use is for facts or "objective" information. You expect your reader to simply trust these outright, so they must be widely accepted in your field as credible sources for facts and information.
Exhibit sources are those you analyze in your essay, ultimately for evidence to help you sustain your claims and deal with counter-claims. Your analysis of these sources—through detailed description, quantitative analysis, or other methods—will likely constitute the bulk of your research essay. These are your most important "primary sources."
Argument sources are ones you draw on for key claims, concepts (with stipulated definitions), and theories you are using and responding to in your essay. In many fields, these will be considered your most important "secondary sources." Most of these will be academic sources (academic journal articles, books or book chapters, essays in anthologies, dissertations, master's theses, etc.). Your essay might be doing any combination of forwarding (applying, extending, revising) or countering (rebutting, refuting, delineating) these arguments.
Method sources are those you use for the methods they model, especially in cases where the method itself is unique, innovative, or particularly applicable to your project. For example, you might cite and describe a certain quantitative method, adapting it for your own purposes in your essay. You might also consider as "method" sources those from which you derive your own mode of questioning, way of thinking, or style of writing. Sources influential in these more subtle ways are sometimes noted in acknowledgements or epigraphs rather than citations.
