Citing your sources is about far more than just formatting references according to some obscure set of rules. Citations serve several important functions:
So, now that we know WHY we need to cite our sources, what do we actually need to cite? This short video explains.
You have an article by Sam Smith. In it, he constantly refers to an article by Jill Jones. It can be very tempting to cite Jones's article in your paper on the strength of Smith's description of her work, even though you don't have copy of her article and haven't read her work directly.
THAT IS A BAD IDEA!
If you cite Jones's article, you're telling your readers that you read it and understood it. You may get away with it, but why risk it? What if somebody asks about another part of Jones's paper? Then again, Smith may have misunderstood what Jones was writing about -- or worse, he may have deliberately misrepresented what Jones wrote. In that case, anybody who reads your paper will think that YOU misunderstood her article or are lying about what she said. In any case, indicating that you read a paper when you didn't is clearly academic misconduct and could land you in a world of hurt.
For those reasons, it's always best to actually read all the articles you reference in your papers. However, there are many reasons why that isn't always possible, so there is a way to INDIRECTLY cite a paper. That's what's called "citing a source within a source." Here's how it works:
In this way, you make it clear that you did not read Jones's article yourself but are relying on Smith's interpretation instead. Any errors or misrepresentations of Jones's paper are clearly on him, not on you.