Plagiarism is the practice of claiming, or implying, original authorship or incorporating material from someone else's written or creative work, in whole or in part, into one's own without adequate acknowledgement.
The above sentence is an example of plagiarism! I took this definition from Wikipedia's page on plagiarism.
You have plagiarized if you pass off someone else's work as your own. You can claim credit only for your own ideas and not someone else's. If you use their ideas, you have to cite the source. Simply put, plagiarism is avoided by appropriately citing your sources.
Does that mean that it is not plagiarism in the above example because I cited Wikipedia, the original source of that quote?
Well, not yet! Had I paraphrased the above quote in my own words, then citing the Wikipedia source would suffice. I have picked up that quote verbatim. Therefore, the quote has to be enclosed within quotation marks (i.e.: " ").
The only place that does not require citation is if the thing you are discussing is common knowledge in that community. For example, you don't have to cite Newton while talking about gravity, nor do you have to cite vant Hoff or Arrhenius before using the Arrhenius equation for computing rate of reactions.
Plagiarism is a serious ethical violation and is taken extremely seriously in the scientific community. Plagiarism therefore has serious consequences. I know of several instances of students kicked out of a university for plagiarizing their technical reports for course-work. There have been documented cases of researchers barred from publishing in certain journals and tenured faculty losing their jobs due to plagiarism.