Sometimes you may need to host the same or very similar content on multiple domains. As an example you might want to have your blog on www.yourblog.com and feature some of the posts on your personal site as well on www.allaboutyou.com. Google calls this 'Duplicate Content' and can cause unexpected results to your ranking. To avoid being penalised for duplicate content you can use 'Canonical Urls'.
Canonical Urls are set as a link tag in the individual pages of your site to tell search engines like Google which url should be considered the 'primary' url if duplicate content is found. These are often used for similar content on the same domain (e.g. site.com/products/myproduct and site.com/products/category/technology/myproduct), but can also be used for cross-domain content. To achieve this place the link tag shown below into the head of your page:
<link rel="canonical" href="http://www.yoursite.com/path/to/page"/>
Link paths can be relative or absolute, though it is usually recommended to use absolute paths to minimise the potential for confusion.
To read more about managing duplicate content for google SEO ranking take a look at their official blog.