Everything after the question mark in a url is called the querystring. It is the set of variables passed to a server-side script page, to be used for whatever purpose. More than one of these are concatenated together using an ampersand. You’ll see them in the vast majority of urls nowadays. Whether they are generated programatically or entered manually depends totally on your particular use.
You might consider going to one of the major booksellers and grabbing an O’Reilly or QUE book for handy reference.