Plagiarism is the unacknowledged and inappropriate use of the ideas or wording of another writer. Because plagiarism corrupts values in which the university community is fundamentally committed – the pursuit of knowledge, intellectual honesty – plagiarism is considered a grave violation of academic integrity and the sanctions against it are correspondingly severe. Plagiarism can be characterized as "academic theft."
The bottom line is that it is better safe than sorry. If you using someone else's idea, give them credit.
Here are some links that will give you more information about plagiarism and what you can do to avoid it:
The Student Guide to Avoiding Plagiarism by Sandra Rhoten, Associate Dean,