What exactly does it mean to plagiarize? According to the American Heritage Dicitonary, to plagiarize is to "reproduce or otherwise use (the words, ideas, or other work of another) as one's own or without attribution."
So, obviously it's not OK to copy word-for-word the work of another without proper citation. Take a look at Cornell's Guide to Academic Integrity (esp. pages 16-28) for some more examples that might be harder to spot. Here are a few: