A webmaster is a free service by Google which provides free Indexing data, backlinks information, crawl errors, search queries, CTR, website malware errors and submits the XML sitemap. It is a collection of SEO tools that allow you to control your site in Google as well as allows Google to communicate with webmasters. For example, if anything goes wrong like crawling mistakes, plenty of 404 pages, manual penalties, and malware identified, Google will speak to you through this tool. If you use GWT, you don’t need to use some other costlier tools. So, it is a free toolset that helps you understand what is going on with your site by providing useful information about your site.
