What is the difference between a snapback and a flat peak?

Snapback refers to the size-selectable 'poppers' at the rear of a hat, which make a snapping sound when you open and close them to adjust the fitting size. Other cap fitting types are 'fitted' (which come in fixed sizes such as 5 1/4 inch), FlexiFit (slightly stretchy, one size fits all type) or strapback (with a catch mechanism allowing you to fine-tune the fitting).

Flat peak refers to caps that are sold with a flat peak as opposed to a curved, formed peak.

So it is possible to have a flat peak or a curved peak with any of these fitting types.