My sites rank very well in msn usually top 10 for all keyphrases, and the same in google once I have some inbound links. But Yahoo my sites are #100+. What does Yahoo focus on for SEO that's so different? Thanks

Yahoo and Google's algorithms are more or less at odds and it is common for a site to rank well in one and not the other. Making your site friendlier to Yahoo usually comes at the expense of ranking well in Google. If you rank well in Google I wouldn't change anything and simply promote your site as normal. Eventually Yahoo will catch up.

Careful selection of keywords, incoming links, title tags, header and keyword-rich content are of course going to be as important to Yahoo as they are to Google.
Yahoo cares about page descriptions. Google usually composes its own page description based on the text of the first paragraph of the indexed page. Yahoo wants you to have a unique page description and you should diligently follow their advice.
Use a "description" meta-tag and write your description accurately and carefully. After the title, the description is the most important draw for users. Make sure the document title and description attract the interest of the user but also fit the content on your site.
Use a "keyword" meta-tag to list key words for the document. Use a distinct list of keywords that relate to the specific page on your site instead of using one broad set of keywords for every page.”

Yahoo does not use meta tags in their search engine rankings. They do read them but they won't affect your rankings. You should still make good meta tags but don't do this expecting a boost in the SERPs. Focus on your content and inbound links.

Yeah inbound links is definetely where im lacking. All my sites rank in the top 10 on msn without any inbound or backlinks, I added 1 backlink to one site and a couple to another and now im #2 on google for the keyphrases. I'll research and add some quality links that should hopefully do it.

> Google usually composes its own page description based on the text of the first paragraph of the indexed page

Actually, the abstract you see in the Google search results is usually the paragraph containing the keywords searched for.

Or the description from the site's DMOZ listing or the site's meta description tag.

So when is which?

Hard to say for sure. It looks like Google goes in this order most of the time:

1) Dmoz description (usually only affects the home page)
2) Snippet from the web page
3) Meta description tag

If you say so.