What is Google webmaster tool?

[Jashandeep]

Hi there !
Can anyone tell me that what is the Google Webmaster tool?
What are the various responsibilities of this tool?

[bposervices]

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.

[AllenSantiago]

1) It shows traffic for every Keyword indivisibly
2) Indexing status
3) Submit a sitemap
4) Find the total number of internal and external links of websites

[markdev]

Google Webmaster Tools provides you with detailed reports about your pages' visibility on Google. It gives you details like View which of your pages are included in Google's index,See any errors encountered while crawling your site, Find search queries that list your site as a result,Find out which sites link to yours .