Maybe Lucius was right and you really don't understand what plagiarism is, so let me explain you.

Every time you are copying someone else's work without providing link to the original source, you are plagiarizing. Furthermore, when you are copying someone else's text  you are supposed to put those parts inside quotation marks so it is immediately obvious to everyone which are your own words and which are copied parts.

An example: You stumbled upon an interesting article and you want to share it with bitcointalk members. Proper way is to copy paragraph or two (and properly format it using quotation marks), share the link to the article and then you leave your own personal opinion on the matter. There is no need to copy whole article, as those interested can always read the article on the website.