It is never right to copy someone else's words and then paste them in a document as if they were your own! That is called plagiarism. It's stealing from someone else, then probably lying about having done it. In the end, puts you in a bad light. Plagiarism is not a crime, but it is not considered ethical. In most academic situations, you can even have consequences such as not receiving a grade if you have taken someone else's work as your own. In some higher educational institutions, such as Universities, you could even be expelled!

It is acceptable, however to link to someone else's site and give a comment on why you think this is a good example of what you are talking about. You can also "quote" and give reference to what someone else has said.

For example on you can find this list of when plagiarism has occurred:

"All of the following are considered plagiarism:
  • turning in someone else's work as your own
  • copying words or ideas from someone else without giving credit
  • failing to put a quotation in quotation marks
  • giving incorrect information about the source of a quotation
  • changing words but copying the sentence structure of a source without giving credit
  • copying so many words or ideas from a source that it makes up the majority of your work, whether you give credit or not."

Follow this link to read more about this topic and how it can be prevented.