What you're getting back is a native Javascript data structure (JSON stands for Javascript Object Notation. I'm not sure I understand your question either... are you asking what is happening here?
What you're doing right now is iterating over the properties of the data
object you got back (which is in the JSON format). Are you expecting a list of user
objects of some kind? Each one having a description
attribute?
EDIT
Ok, so after reading your comment I think I've figured out your problem - since AJAX is asynchronous, you can only work on the data once the request has completed. That's why you're getting back undefined, because the browser continues executing the code without waiting for the request to complete. What you need to do here is work with the data in your callback:
function myFunction(productNo)
{
$.getJSON(
"processors/process_1.php",
{ productNo: 'ABC' },
function(data) {
$.each(data, function(i, user){
alert (user.description)
}
}
)
}