Open paths need to know how to draw the ends of the stroke. For caps, again, there are three options available, which are somewhat similar to the three join types:
Butt cap
Bluntly cuts the stroke, perpendicular to the stroke direction, right at the end node of the path.
Round cap
Adds a semicircular blob that smoothly rounds the end of the stroke.
Square cap
Adds a half-square blob to the end of the stroke.
Both round and square caps make the open path longer by its full width, with a half-stroke-width cap added to each end. Only with butt caps (which essentially means no caps), the path is as long as the distance between its end nodes.
The effect of join and cap options is most visible on wide strokes; for strokes that render at 1 px or narrower, they make very little visual difference (except possibly for long miters).