It's very easy to end up a victim of plagiarism and not even know it, or to even plagiarize someone else's work without meaning to, so it's best that you first know what it is. I'll start the discussion with a few questions and answers here:

"What Is Plagiarism?"
Plagiarism is taking someone else's words and using them as your own. For example, say you have a book report to do and you don't actually feel like reading the book. So, you look it up online and copy the words from a website and claim them as your own. Some call this "borrowing". It's not -- the problem is far more serious. If you don't use quotation marks (") and explain who wrote it, then it's called plagiarism.

"How Can I Avoid It?"
First of all, you should know right away not to do "cutting and pasting". When writing an Times article, you may not feel like writing the article (just like that book report), and use the ever popular cut-and-paste tool. Don't do that; the Editor will catch you and you'll probably get banned from the Times.

Avoid cut-and-paste, and just REPHRASE. Take a paragraph from the original and rewrite it in your own words. Here is an example from a New York newspaper: Young Billy Joe was murdered and thrown into the river last Saturday.

One way to rephrase would be to say: The police say the murdered body of a boy named Billy Joe was found thrown into the river last Saturday.

Or, you could use quotation marks around the original sentence and state your source: A New York newspaper reported, "Young Billy Joe was murdered and thrown into the river last Saturday."

If you're still not sure what plagiarism is or how to avoid it, I'd recommend you talk to someone like an English teacher. Ask questions!

Please, this is a VERY serious matter.

So long,
Monae929 =)

Source: http://sja.ucdavis.edu/avoid.htm