The query string (the stuff after the "?" in a URL) is a powerful tool for web measurement and data collection.
Because of the stateless nature of HTTP, many sites use query strings in their URLs to pass information from page to page or request to request. The query string is a specific portion of a URI that begins after the URI stem and a question mark (?), as shown in the following example:
www.mysite.com is your
server DNS or (s-dns) name,
is your URI stem and
your query string. The query string in this example contains one query
string, which is composed of a parameter, variable, or name=value
Within the structure of a query string, there may be multiple pairs, each consisting of a variable, followed by an equal sign (=) and then the data or value associated with each variable name. If there are multiple query string variables in your query string, they will be separated by an ampersand (&). Simply put, the query string is a sequence of name=value pairs separated by ampersands (&) as in:
Query strings can be used to carry almost any type of data back to your web servers for a very wide range of purposes. Any query string parameters added to the URL typed into the address bar of the web browser will likely be captured and logged along with the rest of the URL by a responding web server. Finally, certain "unsafe" characters ...