The main difference between a catalog item and an order guide is what happens after you submit them. A catalog item works with the cart where you can add multiple and then checkout. On the back end it creates a request, request item, and possibly approvals and tasks depending on its workflow. A record producer is ordered one at a time and creates one record on the back end, most commonly an incident. An order guide is used to guide the user into ordering multiple items.

Introduction to Service Catalog - ServiceNow Wiki