Each time you visit a website a variety of details about your user experience are communicated to the servers that host the site by way of a set of headers. These headers contain tokens which indicate to the server the best way to provide requested  content to the user, based on their operating system, browser and which version of the software being used. This information, called the user-agent string, ensures that a server accomodates traffic from a variety of devices, (mobile, tablet etc) browsers and software.

You can view your own User Agent String in a variety of ways. A simple tool is www.whatsmyuseragent.com. For help understanding the elements within a string visit, www.useragentstring.com.