Any and all sources, particularly when we are quoting from another website, journal article, book etc. must be properly attributed to the source.
Attributions can take the following forms (and folks are welcome to add to this):
A link to the web source, this should contain the name of the article (in case of dead links later on) and attribution to the author and/or organisation.
A full reference for non-online resources (yes, they do exist).
Failure to attribute to the resources is outright plagiarism and is not only unethical, it is in many cases, illegal. At the very least, it casts a very negative light over this site.
Basically, if you did not write it, even if you did - give a full link, hence full credit to who wrote it and where the information came from.
All members need to keep an eye out for this activity - if you see it, downvote and flag it. It is almost tacit approval if people upvote a plagiarised question or answer.
Anecdotal side note: I have had my work plagiarised before, I can tell you from the receiving end of that, it is a awful feeling.