The terms duvet and comforter are often used interchangeably as they both have a quilted design filling, but a comforter is used on its own while a duvet insert is intended to go inside a duvet cover. They're typically differentiated by loops on the corner of the duvet to keep its cover in place.
A duvet insert lets you get personal about the fill type and weight, whereas a comforter may only be available in one fabrication. Really, so long as you can machine wash your duvet insert, it can definitely be used as a comforter, sans cover. If you're looking at a duvet insert you have to dry clean or spot clean, you'll want to use a duvet cover for ease.