Smartphone density is increasing very rapidly and smart youngster's are now prefer those devices as their digital friends. The browsing experience on these phones are a bit different. It is not like a desktop browsing where they have a big screen, and powerful processors. So, many websites now started showing smartphone specific content to their viewers when browsed on such mobile devices.
To index smartphone specific contents in Google search, Googlebot-Mobile now crawls web with a smartphone user-agent in addition to its previous feature phone user-agents. Google says that this will provide a better search experience for smartphone users. The new user agent string introduced is this:
Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_1 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8B117 Safari/6531.22.7 (compatible; Googlebot-Mobile/2.1; +http://www.google.com/bot.html)
Two Feature Phones Googlebot-Mobile user-agent strings are also functional:
SAMSUNG-SGH-E250/1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 UP.Browser/126.96.36.199.c.1.101 (GUI) MMP/2.0 (compatible; Googlebot-Mobile/2.1; +http://www.google.com/bot.html)
DoCoMo/2.0 N905i(c100;TB;W24H16) (compatible; Googlebot-Mobile/2.1; +http://www.google.com/bot.html)
A New Story Introduced with this:
With the introduction of this smartphone specific user-agent, a new feature has been proposed which will exclusively be used for Skip Redirect for Smartphone-Optimized Pages. It means that when Googlebot discovers a URL in search results that redirects smartphone users to another URL, for serving smartphone-optimized content, Googlebot will directly use the final destination URL as the link target in search results. Intermediate redirecting will be avoided.
This removes the extra latency caused by redirection and may save 0.4-1 seconds on average when visiting landing page for such search results.