boss

2 (Architecture) an ornamental carving at the point where the ribs in a ceiling cross.

— ORIGIN Old French boce.

rosette:

2 a design or object resembling a rose.
— ORIGIN French, ‘little rose’.


These are typical rosettes:
http://www.wishihadthat.com/index.asp?PageAction=VIEWCATS&Category=797
the most common one is probably the one in the third row, second column. This is the typical rosette seen where the horizontal and vertical moldings around doors and windows meet. It's basically a way of hiding the joint in the wood by distracting your eye from it.

These ( http://snipurl.com/101rk ) are some pictures of architectural bosses which are not rosettes.

So a boss can be a boss without being a rosette, but in architecture rosettes are sometimes bosses, though they can certainly be used in other decorations. Many military items have rosettes on them.

TEd