The basic advice is to avoid using multiple domains pointing to a same (primary) website unless you setup 301 redirects to the primary website.

Buying multiple domains and pointing to the same website or clones of the same website may actually decrease your page rank and likehood of being found on search engines. But for certian cases, there are techniques for muliple domains that can be used without degrading your rankings.

Keep in mind, you should never try to trick the Search Engines by buying muliple domain names (or any other method). This is NOT good SEO. People who buy muliple domain names for the sole reason to flood the search engines will eventually get caught out and their sites banned. Honesty and "Good Value Content" will pay out in the long term.

Protecting Your Brand

It is sometimes prudent to buy muliple domains for your business to protect your band ie:

  • bottomupwebs.com.au (primary)
  • bottomupwebs.com.au
  • bottomupwebs.net
  • bottomup.com

For this case, use the 301 redirect on all the secondary domains

Targeting A Country

It is common to have muliple domains targeting muliple countries.

If you have muliple country "tweaked" websites that have duplicated content but use their own country codes .uk, .au, nz, and have their own contact page or localised spelling - then use the rel="canonical" tag.

The rel="canonical" tag is placed on the duplicate sites to point to the main site. Search engines can then read what is going on and won't penalise you for  doing so.

For example, on bottomupwebs.com, in thesection place the code...

For more info in Mulit-Regional Sites: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=182192

For more on rel="canonical" tag:  http://support.google.com/webmasters/bin/answer.py?hl=en&answer=139394

Targeting A Language

Translated content is not considered as "duplicate content", so you can do this without penalty in theory if the language is different on each of the muliple domains.

Ideally the recommendation is to use a subdomain for each language on the primary domain.

If teh languages are similar (ie: Australian, British, American) then best to use the rel="canonical" tag.

Targeting Keywords

Use 301 redirects on the secondary domains to take users to the primary domain. 301 are permament redirects (302 redirects are temporary). Search engines will ignore these domains, but if people type these words directly into the URL box of their browsers, they will land on your site.

Buying additional domains names purely for SEO is not a good practice. Building separate websites for each domain name is an option - but these website require unique content.