jForms generate forms from XML descriptive files. Every form is describe in an XML file, using a simple grammar between say HTML and XForms.
Those XML files reside in forms folder of modules et must follow this name scheme foo.form.xml. foo is the identifier of the form. You choose it of course. This identifier is later used in selectors and jForms API.
Example: "myModule~product" refers to myModule/forms/product.form.xml.