Probably not, assuming you’re not deliberately pretending you wrote a passage that you’ve in fact stolen from another writer.
If you refer in your own writing to another author’s work to support a point or illustrate an example, and you properly cite that source (at minimum: author and book/essay/article title), you’ll be fine.
For example, always-insightful author Seth Godin tell us, in his recent blog post “Simple thoughts about fair use,” that you can quote hundreds of words from a book without having to obtain permission from the author or publisher. All you need to do is credit the source…. “According to David Ogilvy, in his great book Confessions of an Advertising Man….” That should do the trick.
For a very good definition of plagiarism, copyright law and related topics, visit plagiarism.org.