Google Search Console (previously 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. As of May 20, 2015, Google rebranded Google Webmaster Tools as Google Search Console. It has tools that let webmasters:
Submit and check a sitemap.
Check and set the crawl rate, and view statistics about when Googlebot accesses a particular site.
Write and check a robots.txt file to help discover pages that are blocked in robots.txt accidentally.
List internal and external pages that link to the site.
Get a list of links which Googlebot had difficulty crawling, including the error that Googlebot received when accessing the URLs in question.
See what keyword searches on Google led to the site being listed in the SERPs, and the click through rates of such listings. (Previously named 'Search Queries'; rebranded May 20, 2015 to 'Search Analytics' with extended filter possibilities for devices, search types and date periods).
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.
Highlight to Google Search elements of structured data which are used to enrich search hit entries (released in December 2012 as Google Data Highlighter).
Demote Sitelinks for certain search results.