Content is King. It's a rule that I learned when I worked in the games industry a while ago. While technology is great, content is more important. It's content that draws people in (would you sign up for Facebook if no one else was there?). It's content that keeps people engaged (would you stay on Facebook if your newsfeed was empty?). And Content is King is something that Google also values.
But on the web, it's really hard to protect your content. Or more accurately, it's really easy for someone else to copy your content. Image search makes it super easy to find and download a photo (making those CDs we once bought of clipart a mere memory for people over 30 and something people under 30 would just not understand). And copying text from a page is also easy.
Right now, I'm in the process of getting content created for a blog that I am creating for my startup. I'll be using Elance to hire writers for original content. But how do I know if the content that I am getting is original? That's where CopyScape comes in. CopyScape will look at any page on the web and find pages that have the same words. It's a quick way to see if the article you just received is original. It's also a way to see if anyone has 'leveraged' your content.
Here's an example:
The top result has 48 matching words with my article, which is only 5% of the page. A quick visual check of the page shows that both sites are using the same commenting form. So, I now know that no one has copied and published my article.
CopyScape provides a manual URL check for free. Just put in any URL and it will show the results of the pages that have the most words in common with that URL. They have paid models for more automated checking.
Check it out and see if your content is unique on the web. If it is not, Google may penalize you for having duplicate content.