While the company has long used cookies, IP address logging and other methods of identifying visitors to its online properties, they are relatively coarse measures and easily defeated (eg, by blocking cookies altogether - a tall order these days - or at least by frequently clearing out the cookies).
Many of Google's more recent services and features encourage you to identify yourself to the company. Examples include Gmail (would you let someone else use your browser while you're logged in to your mail account) and the iGoogle home page.
The company's latest search improvement requires users to be signed in if they are to see any benefit.
Known as +1 (as in an online endorsement or thumbs-up), search results will be tagged with the names of people you know that have recommended that page. But - understandably - if you're not logged in, Google isn't sure who you are, and can't pass on the recommendations.
If you are signed in, how does Google know who you know? Find out on page 2.
The Google Dashboard will be modified to show your connections.
To give +1s to pages you like, you must create or upgrade your Google Profile (which will also show all your +1s so you can delete them if that becomes appropriate), and you must be logged in to your Google account to see or give +1s.
While the +1 button will initially only appear in organic search results, Google plans to add it to ads on Google.com and allow AdSense publishers to put +1 buttons on their own pages. In time, +1s may become one of the factors that affect the ranking of individual pages.
+1 is initially being implemented in English on Google.com, but will be extended to other languages and regional/national Google sites.