Google Webmaster Tools is a no-charge web service by Google for webmasters. It allows webmasters to check indexing status and optimize visibility of their websites.

It has tools that let the webmasters:

Submit and check a sitemap
Check and set the crawl rate, and view statistics about how Googlebot accesses a particular site
Generate and check a robots.txt file. It also helps to discover pages that are blocked in robots.txt by chance.
List internal and external pages that link to the site
See what keyword searches on Google led to the site being listed in the SERPs, and the click through rates of such listings
View statistics about how Google indexes the site, and if it found any errors while doing it
Set a preferred domain (e.g. prefer example.com over www.example.com or vice versa), which determines how the site URL is displayed in SERPs

Contents

1 Criticism and controversy
2 Google Webmaster Tool VS Google Analytics
3 See also
4 References
5 External links

Criticism and controversy

The list of inbound links on Google Webmaster Tools is generally much larger than the list of inbound links that can be discovered using the link:somewebsite.com search query on Google itself. Google is tight lipped about the discrepancy. The list on Google Webmaster Tools includes nofollow links that do not convey search engine optimization authority to the linked site. On the other hand, the list of links generated with a link:somewebsite.com type query are deemed by Google to be ‘important’ links in a controversial way. Google Webmaster Tools, as well as the Google index, seems to routinely ignore link spam.
Google Webmaster Tool VS Google Analytics

Google Webmaster Tool shows traffic for each keyword separately, it gives more information about website performance according to Google search query. This tool does not cover search results from other search engines, such as Bing, Yahoo, or Baidu.

Google Analytics shows total traffic of a website such as clicks to your site regardless of where they came from and what search terms were used. It shows whatever search terms brought up your site in the listings. While your keywords might be there too, many will probably be specific terms or long-tail search terms. There are probably even search terms where Google offered your site even though they don’t see a perfect match. If Google thinks the searcher meant something else, maybe they displayed your site for that word or term as a suitable match even though that word is not listed in Webmaster Tools as even being on your site.
See also

Google Insights for Search
Google Analytics

References