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Communicating with a server may involve sending that server information.
Generally, communicating with servers is accomplished over the open internet using HTTP requests.
There are two broad ways of accomplishing this: embedding information as a query string within the URL being visited or including information in the body of the HTTP request in the form of JSON or XML.
This page covers the former.
A '?' after a URL signifies the start of the list of arguments as key/value pairs. Example by googling “hello” (note: in the top bar of google chrome; if done from google.com returns something different):
A key (or variable) is denoted by the text to the left of an equals sign, while its value sits on the right. The same URL chunk with whitespace:
?q=hello& rlz=1C1CHMO_enUS567US567& oq=hello& aqs=chrome..69i57j69i60l2j69i59l2j69i60.554j0j8