If you should give credit to an author, THEN DO! When you don't you are committing plagiarism...and it's a serious crime.

Plagiarism is presenting the words or ideas of someone else without properly acknowledging their source. Failing to acknowledge the source implies that the words or ideas are your creation.

You need to credit an author when you quote something he/she wrote or said, and when you summarize or paraphrase information from books, articles or the Web.