In the ServiceNow Service Catalog, the actual form structure-the input fields, checkboxes, and drop-downs that a user interacts with-is built using Variables.
While a standard ServiceNow form is defined by "Fields" on a table, a Catalog Item form is defined by a collection of Variables. These variables determine what data is collected from the user. To further define the "structure" or layout, these Variables can be grouped into Containers (which create sections, columns, or split layouts) and Variable Sets (groups of variables reused across multiple items).
Order Guides are used to group multiple distinct catalog items into a single request process, not to define the structure of a single item's form.
Catalog Description is merely the text field describing the item.