Cecil O. Samuelson discussed plagiarism in his 2004 address to the student body at BYU:
I suppose intent goes into it. If you're shopping and you stick something in your pocket and you honestly intended to pay for it along with everything else in your shopping cart, but forgot, and walked out of the store with it, is that dishonest? I suppose not, if it was an honest mistake. Accidental stealing is still stealing, though. And you would have a tough time convincing anyone that you did not intend to steal the item when you stuck it in your pocket. We don't usually stuff items we intend to pay for in our pockets when we shop.The definition is straightforward, but the practicalities seem to trip many people—including some of you. Basically, to plagiarize is to take or pass off someone else’s words, ideas, or work as your own. As with most infractions, there are both blatant and subtle examples of plagiarism. To lift pages or paragraphs from published works or from the efforts of a classmate or anyone else without permission or citation constitutes plagiarism. Of course it is appropriate and often helpful to quote the works of others, but it is always necessary to give clear and adequate attribution to your sources. Some believe that using materials from the Internet or obscure resources loosens the rules. It does not! Some consider it appropriate to change a few words or the sequence of a few sentences or paragraphs and then claim the product as original work. Again, think of the basic definition.
When you copy and paste sentences written by other people, and put them in a post right alongside your own writing, with no indication that one sentence is your work and the next is not, the reader is deceived into thinking it is all your work. Maybe it was sincerely not your intent to deceive, but the deception still happened. And you would have a tough time convincing people it was not intentional if you made no effort to disclose the quoted material.
Obviously we are not in college here. None of us is getting academic credit for anything we write. This is just an internet message board and the stakes couldn't be lower. To me, plagiarizing in an environment like this is like cheating in a board game you're playing with friends. The other players might be offended or they might not care at all, depending on how seriously they take their board gaming.