Let's be honest, plagiarism, that ugly "P" word, is still an issue today; and not just for students who don't understand that their teacher can check for such things through magical devices called computers, but also for the working adult who is supposed to be the example.
Plagiarism is the internet presence equivalent of somebody passing gas in a small room full of people; most of the time, eye contact is absent, and everyone wants to avoid the subject at all costs.
The Gassy Elephant In The Room
Speaking of avoiding subjects, guess what we're NOT discussing today? The moral implications of plagiarism. Too often, people take to their soapbox on topics like this, when honestly, it's a worn down topic with too much hypocrisy to keep track of.
The nice thing about plagiarism is that Google doesn't care about the why; they just penalize you. Google takes morality out of it and provides real penalties to you and to your web presence. Google has 200 ranking factors for your site and information. Do they release all those factors? No, they do not, but they give some, and they are pretty clear.
Ranking Factors That Could Be Hurting You
Plagiarism: Blatantly copying another person or company's content and sticking your links in not only doesn't look good, it quite literally hurts your ranking in Google. They find it, and then you later see yourself dropping down the page. So if there weren't reasons to stop before, there are now.
Spun Content: Are you reusing articles? Consider this a branch off the Plagiarism tree. It's not a bad thought, initially at least. Except that if you paid someone to write articles for you, and they just happen to be scamming you with content that they have written before and exists already on the web. Or if you have content that is too similar on your own site, even if it was written a while ago. Talking about shooting yourself in the foot.
There are a lot of additional sources that show other important factors into the SEO ranking of your site, but it's important to know the value of original content. Google, like a kid on a parade float throwing candy at your face, rewards you for original content. On the other hand, Google penalizes you for copying or recycling content.
So now that you know what happens, you can go out and be creative, and feel good knowing that Google loves your creative, and original content.