Buckle carriers, (also known as "soft-structured" carriers, or SSC's) are very much like frameless backpacks, with looped shoulder straps and a waist strap with buckle for added support. They are designed so that your baby is held upright against your chest, back or hip. These types of carrier tend to be a better option for babies older than 4 months, as they usually require your baby to be able to support their own head and place their legs around your waist; however infant inserts are available for younger travellers.
Buckle carriers are particularly good for unassisted, simple back carries, as their structured panel means they hold your little one in more securely than wraps. They are also easier to get babies in and out of. We often find the men in the family are more open to using them as well!