Wikipedia:Classification of sources
 | The following is a proposed Wikipedia policy, guideline, or process. The proposal may still be in development, under discussion, or in the process of gathering consensus for adoption. |
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This (guideline/policy/explanation/something) gives guidance regarding the classification of different sources as they are used for wikipedia policies, guidelines, and discussions. Uses within articles should be based on the broader definitions found in the relevant articles (primary source, secondary source, tertiary source.
There are multiple way to classify sources, the most important to wikipedia being the primary/secondary/tertiary distinction, and the concept of third part sources.
Primary, secondary, and tertiary sources
A primary source is a photograph, video, eyewitness statement (in the context of, say history), or a TV show itself, a script, a novel, etc. A secondary source is anything that isn't primary. A tertiary source is a special case of secondary sources which do not use any primary sources themselves.
Thus, if someone edits wikipedia with information they had gained first-hand, that would be unsourced, and the article would itself be a primary source. If they directly quote an eyewitness or a text, then that's sourced from a primary source, and the article is a secondary source. If the only directly used sources are secondary (including tertiary sources), then the article is a tertiary source (often seen as the ideal for wikipedia).
The main purpose behind the distinction (on wikipedia) is the concept of original research - the composition of a primary source, or the synthesis of primary sources into a secondary (non-tertiary) source are research. If this is done for the article, then that is original research. Thus, if a person watches a TV show, reads a book, watches a film, or any similar activity and then writes a plot summary, that's original research. If they find one or more plot summaries elsewhere and reference them, then it's not original research, it's synthesis or composition of secondary sources.
Journal articles aren't a primary source, generally speaking - raw experimental data is a primary source. Primary sources are typically (but not always) devoid of interpretation. Interpretation is added through research in the synthesis of the secondary source. WP:NOR is saying that we don't do interpretation, we do synthesis compilation.
News reports may or may not be a primary source.
Third-party sources
In short, a source is third-party if it's not created or influenced by the subject it covers.