Well, if you're using a form with method GET, the data that's entered is added to the query string. For example:
HTML:
<form action="form.php" method="post">
<input type="text" value="value1" name="input1" />
<input type="text" value="value2" name="input2" />
<input type="submit" value="submit" />
</form>
If you press the submit-button (without changing the values), the user would get referred to form.php?input1=value1&input2=value2
Of course, instead of using a form to add the query string, you can also use links with query strings or just type it in the address bar of your browser...
The use of these query strings is like this:
if you've got the above example (form.php?input1=value1&input2=value2), you can access the values of input1 and input2 in php: $_GET['input1'] and $_GET['input2'].
Note: method GET is not really secure, as you can see what you typed in in the address bar of your browser... don't try to send passwords with this... use method POST instead.