A user agent is the raw information relating to the visitor connecting to your site. Whenever someone, or some thing, connects to your website, they are passing information pertaining to the agent used to connect to your site. While most see Internet Explorer as the method of connection, a Web server sees something like: mozilla/4.0 (compatible; msie 6.0; windows nt 5.0). This is the user agent. Web browsers, search engine spiders and robots, and various website monitoring tools all have specific user agents.
This report displays information about the user agent and is most commonly used by advanced users and website developers. For example, this report can be used to determine the proportion of users that are using traditional Web browsers versus accessing your site via mobile phone or other devices.
In addition, the user agent string is one of the criteria by which Web crawlers can be excluded from certain pages or parts of a website using the robots.txt file. This allows webmasters to request exclusion of individual webpages or directories from the data gathered by a particular crawler, or when a particular crawler is using up too much bandwidth, by requesting that crawler not to visit certain pages.