Search engines measure a website's value and relevance by analyzing the links to the site from other websites. The resulting “link popularity” is a measure of the number and quality of links to a website. It is an integral part of a website's ranking in search engines.
When you link from your website to other relevant and authoritative websites, you're adding value and providing a great service to searchers. We're in the process of performing some tests to determine (with more certainty) whether outbound links help or hurt your website's rankings and SEO.
In general, by including a hyperlink, you are not actually copying. It is generally fine to provide a hyperlink to another website as long as that link is not a site which is known to contain infringing material. The law is not clear on deep linking.