According to Google the best way to get other sites to create relevant links to yours is to create unique, relevant content that can quickly gain popularity in the Internet community.

Google webmaster central nicely sums up the relevency part for link building.

“Your site’s ranking in Google search results is partly based on analysis of those sites that link to you. The quantity, quality, and relevance of links count towards your rating. The sites that link to you can provide context about the subject matter of your site, and can indicate its quality and popularity. However, some webmasters engage in link exchange schemes and build partner pages exclusively for the sake of cross-linking, disregarding the quality of the links, the sources, and the long-term impact it will have on their sites. This is in violation of Google’s webmaster guidelines  and can negatively impact your site’s ranking in search results.

It is not only the number of links you have pointing to your site that matters, but also the quality and relevance of those links.”

In one of his blog posts, Matt Cutts said:

“Canonicalization is the process of picking the best url when there are several choices, and it usually refers to home pages.”

Lets say your home page can be accesses at -

http://www.example.com/

http://example.com/

People will find the same information at above two url’s. They won’t know the difference, right ? But search engines will. Googlebot sees the above two URLs as two different pages on the web.

Search engines can index both www and non www versions of your domain . Therefore, it’s best for SEO purposes to stick with one version of your domain and make sure all backlinks point to that version. The www version is recommended because most sites will link to you using that version anyway.

Once you have decided your preferred version ( www or non www ), you should implement 301 permanent redirects at the server end to ensure your preferred version is served no matter which version is typed in. You can also set your preferred domain within Google Webmaster Tools.

As far as ranking is concerned, there is no such preference from any search engine. The search engines will rank whichever URL has the highest number of quality links pointing to it.