Pagerank is passed from one site to another via links. Whenever one site refers someone to another site, a little bit of pagerank is passed on from the page providing the link to the page receiving the link, provided that the link is not specified as being a nofollow link.
If you have a page with a lot of outbound links, you will have a much harder time ranking that page than if you would if you had fewer outbound links.
To avoid what is commonly called "bleeding pagerank", I recommend including the following attribute in links to other sites unless you specifically want to help them build pagerank:
- <a href="URL" rel="nofollow">Link-Name</a>
Additionally, you might want to also consider having links to external sites open in a new window so that your site will still be displayed once a visitor closes the external site. To do so, include the target attribute as follows:
- <a href="URL" rel="nofollow" target="_blank">Link-Name</a>
By using both together, you avoid passing your pagerank on and ensure that visitors who follow external links have a better chance of comingback to your site.