Oh my. What has CGI has to do with the query component of an URI? https://tools.ietf.org/html/rfc3986#section-3.4 They are definitely NOT known as CGI parameters at least in the ten years I am a senior web developer (sometimes working on Top 100 sites) I never heard them called such. Googling "CGI parameters" finds only 21 400 documents which indicates it is definitely not a widely used phrase.

CGI will put those in the QUERY_STRING variable but also you can use PATH_INFO to pass information to a CGI script.

As to what they might contain: anything. There's no standard. Most everyone went to so called "nice URLs" or "pretty URLs" or whatever which -- as I just mentioned -- gets passed in PATH_INFO -- nowadays they almost always contain tracking information. Very often it'll be utm_... which indicates a very lazy team using Google Analytics and Tag Manager. For one (of the many) nicer ways see http://www.lunametrics.com/blog/2014...gn-parameters/

paperwastage's example is not a query either, it's a fragment... and for fragments there's even less standardization, that Google Flights puts key-value pairs in a fragment is an interesting decision, not more. You will find most fragments are not key-value pairs.