Inbound links = links coming INTO your site from other related quality websites
Outbound links = links going OUT from your site to other related quality websites
Everybody hammers on about the importance of getting quality links from related websites focusing on high PR, .gov and .edu sites as the main ones to target (if possible), which is all very important and the main part of an SEO campaign.
Buy what a lot of webmasters and some SEO's forget is the importance of OUTBOUND links from the pages or site you are targeting.
Importance of Validating Your Content With An Outgoing Link
That's right - links that go out from your site to another site, when this topic is brought up some people are concerned they will lose their PR by giving away their link juice. But it is crucial in any well rounded SEO campaign to place a strategic OUTGOING link on the pages you are SEO'ing to some of the most respected authority sites that relate to that page's content. (One to two (2 being maximum)).
The Right Link Will Add SEO Weight
In doing this you allow add more 'SEO weight' to your pages theme, which in turn adds to the importance placed on it by Google for that theme. When placing an 'outbound link' don't add your keywords as the anchor link title, simply the Name of the website resource or even the plain URL as the link. (You keep your keyword links as internal ones).
Just though I would add this as I have found it extremely helpful in improving ranking in the SERP's.