An e-invoice is an invoice issued through the GSTN portal, as opposed to a regular paper invoice. It includes all the required details and information in an electronic format, which can then be shared with customers or other entities via email or on their respective portals. The biggest difference between a regular GST invoice and an e-invoice is that the latter has to be uploaded on the GSTN portal. It also includes a unique e-invoice number generated by the system itself.