The Google "Sandbox" is a purgatorial state, in which a website is spidered by the Google bots, accumulates backlinks and page rank, but does not appear in Google searches for appropriate keywords.

In other words, Google Sandbox is an alleged filter placed on new websites. The result is that a site does not receive good rankings for its most important keywords and keyword phrases. Even with good content, abundant incoming links and strong Google Page Rank, a site is still adversely affected by the Sandbox effect. The Sandbox acts as a de facto probation for sites, possibly to discourage spam sites from rising quickly, getting banned, and repeating the process.

Purpose of Google Sandbox

It is thought that the reason Google created the Sandbox new site filter was to stop spam related sites from adding numerous purchased links--and ranking highly for their keywords from the date of launch. Since Google apparently considers a high number of links pointing to a site from the beginning to be rather suspicious, the links are not considered to be natural. Another possibility is spam sites would use various tactics to rise to the top of the search results, and gain heavy sales prior to being banned for being in violation of Google's Terms of Service; and then repeating the process continually. As a result, new sites are put into a form of probation, usually referred to as the Google Sandbox.

Ways to come out of Google Sandbox

• Pay Per Click There of course is AdWords, Google's pay per click advertising program. If you have a new site and are finding yourself caught in sandbox to where your site will not show well in the Google searches, why not put aside a budget for an AdWords program? With AdWords, you can instantly gain exposure on Google as well as many search and contextual partner sites. This can bring traffic to your site as a direct result of people searching at Google or one of their search partners such as Ask Jeeves, Netscape, AOL as well as others that display AdWords on their sites.
  
  
• Other Search Engines Don't discount traffic from other search engines such as Yahoo, Ask Jeeves and MSN. If you only focus on Google in your SEO strategy, you might miss valuable traffic that you can receive from these other sites, all of which do not seem to have any type of aging filters. Besides that, sites that have well "on the page" search engine optimization seem to do very well in these engines. Yahoo and MSN are lighting fast about finding new or updated content and including it in their index.
  
  Therefore do not neglect optimizing the various elements of you site's pages that these engines factor in to their algorithms - title tags, meta description tags and the actual html text on your pages. If you optimize these elements properly, you will most likely experience very good placement in these engines and as such will gain a good quantity of visitors.
  
  
• Patience Is A Virtue All in all, be patient. Simply accept the fact that if you have a new site, it will take quite awhile before it will rank well in Google. This will allow you to be more at peace with your marketing efforts as well as have the foresight to look at other alternatives.

Reference: searchengineguide.com and Wikipedia - Free encyclopedia