One important consideration when we are evaluating how useful and authoritative a source is in a given context is to consider the source's role, or its relationship to the topic.
Is this source a kind of raw, unfiltered evidence that needs to be analyzed and put into context? Because they provide firsthand evidence of a topic, these types of sources are called primary sources.
Is this source an outside observer who can compile and analyze the evidence and provide an overall, bird's-eye view of the situation? These secondhand compilers, commenters, and analyzers are called secondary sources.
There's a third type of source which aims to make the information and expertise provided by secondary sources accessible to people who are new to the topic, whether by offering a dictionary or a glossary of key terms, or summarizing the various issues that make up the larger discussion about the topic. These types of sources can be called tertiary sources.
Each of these source types is suitable for certain purposes, but inappropriate for others, as a result of the nature of their relationship to the given topic.
Primary sources are sources that provide raw evidence or direct testimony about a topic. A primary source could be an eyewitness account of historical events, or it could be a historical object or a painting.
It is oftentimes the case that a source of information may not have enough credibility to serve as a secondary source, but can be used as a primary source.
(NOTE: There are any number of reasons why a primary source might not be considered 100% reliable just by itself; a major role of secondary sources is to put primary sources in context and evaluate their usefulness and reliability.)
One useful way to think about how to appropriately use primary and secondary sources is to remember: primary sources get analyzed, while secondary sources do the analyzing.
For example, a Youtube video may not have enough authoritativeness to be used as a reliable source of information about a topic, but it can get analyzed by a secondary source as an example of what discourse exists on Youtube about that topic.
(This is an important but subtle distinction: proving something about the topic vs. proving something about the discourse about that topic--not the same thing!)
Secondary sources analyze the events or topic in question from afar, bringing together the evidence from multiple primary sources and other secondary sources to put the topic in context and provide deep analysis.
Examples of secondary sources include:
One problem we constantly run into when dealing with secondary sources is evaluating source credibility/authoritativeness.
In theory, the category of secondary sources could include all kinds of sources that comment on a topic secondhand, including social media, blog posts, newspapers, and Youtube videos.
However, many college instructors when assigning papers will use the term secondary sources as a shorthand for authoritative sources, or--in higher level college classes especially--assume a shared understanding that the only sources that satisfy a requirement for "10 secondary sources" are scholarly sources.
(It is always best to seek clarification from the instructor if you are unclear about an assignment's requirements.)
The authority of a given secondary source (as the ACRL's Framework for Information Literacy for Higher Education reminds us) is always going to be Constructed and Contextual.
In the context of academic writing (or writing for a scholarly audience), it is however possible to create a general ranking/hierarchy of secondary source types based on how authoritative they are considered to be.
Tertiary books are the best kind of book to consult if you are entering into a topic for the first time.
They are called tertiary because they have a third-hand relationship to a given topic: they aren't the immediate witness to what occurred (primary) nor are they analyzing it in maximum depth after the fact (secondary). Instead, tertiary sources take the complexity of primary and secondary discourse and digest it into a more basic form that laypeople (non-experts) can enter into in order to get an introduction to the topic and major issues and subtopics it contains.
Reference works such as encyclopedias, dictionaries, and bibliographies can offer a brief overview of a wide variety of topics you may be interested in researching, quickly bringing you up-to-speed on the history of research (or historiography) of your chosen topic, pointing the way to other important primary and secondary sources, and allowing you engage in the broader conversation about that topic among scholars.
At the same time, instructors are generally not looking for a citation to a tertiary source when they ask for "X number of scholarly articles." While many tertiary sources are produced by scholars (and so, in a technical sense, are scholarly), their relationship to the knowledge created by academic scholarship is to compile and summarize that knowledge, rather than to create new knowledge and understanding (as is the expectation with secondary source scholarly articles and books.)
In academic writing, it is generally considered better to consult (and give credit to) the original source of the arguments and information we wish to engage with. This means (for example) consulting the original research article rather than a summary of it, or the actual book some information is discussed in rather than just a book review.