what is the difference between optimizing a website for google and for yahoo and for msn?

Basically the difference in algorithms (which MSN Google and ASK.com have made public) are that MSN weighs heavily on website content and structure, it also uses AI to determine why a page is consistently visited more than another.
Google tends to weigh external links heavier and relavent content, if you link to a lot of people and they link back you tend to get a better search rank.
ASK.com focuses more mostly on inbound links (who links to your site). If more people have links to your site, then you must have a page worth looking at.
Here's an article to explain it more in depth.
http://www.searchenginejournal.com/ask-j...
by the way, yahoo's kind of fallen stale and you probably shouldn't waste any money on it until they either make a comeback or go the way of AOL.

thanks for the info, Oracle.
Regarding Yahoo, as far as I know they are still the second biggest search engine, but maybe their technology is not getting developed further, I dont know about that.
Is there something in the optimization of a site that specifically works for google, but works against msn or yahoo?

I'll definitely concede that yahoo's still a big player as far as search engines go. but they are definitely taking a wierd path as far as search engines go. Anyways, as long as you're not doing anything like google bombing, the rules for each of the engines either fairly mutually exclusive or mutually beneficial, meaning that certain tricks for optimizing for one site do not hurt your rank in other sites, or more likely help your ranking in other sites.