This page would be a good starting point for reviewing the material you need to understand for this type of situation. In particular read the section immediately following Example #2.

Just to summarize, your query string is based on key->value pairs, and is considered a GET request. PHP automatically stores all values it finds in either a POST or GET request in similarily named superglobals, such as in your case $_GET.

To apply a GET query string, you're going append a question mark, followed by the key->value pairs, such as http://mydomain.com/mypage.php?key=value&this=that (multiple key->value pairs are appended with an ampersand). From within PHP you can access these with $_GET['key'], which would for the previous example return 'value', or $_GET['this'] would return 'that'.

From there, your options are wide open to interpret the values and act upon them.