Interface Process is the Subset of the Conversion Process. It Means , When we migrating the Data from External source to the Oracle Apps, but the data is not in the Form or in the shape like Oracle apps required Data format , so we need to massage this data or to put extra efforts to prepare this data in the form of Oracle Required format data that process is called conversion. Once the data prepared in the format of Oracle apps required format then the Interface process starts to import or migrate the data in Oracle apps.
Conversion most of the Time is the one-time process when we are doing implementation or we are switching the ERP application from other application to Oracle application. But Interface is the continuous on-going task which helps to sync the external applications with the Oracle application.