Since headstock silhouettes are an important branding element, their variety defies common nomenclature. That being said, players and collectors have come up with some general terms to refer to common shapes of headstocks and crests:
Headstocks:
Shield - common 'ukulele profile, generally has a crowned crest and tapers toward the base (Martin, Kala, standard Kanile'a, etc.).
Flyswatter - variation on the shield. Base is as wide as or slightly wider than crest (Riverbend Instruments, Koaloha, etc.). Popular shape among the more "folksy" custom builders. A few alternative terms exist for this shape, but none are coming to mind at present.
Boatpaddle - another variation on the shield, but this time with a wider crest than base.
Snake/snakehead - similar to a flyswatter, but with a more pronounced taper toward the crest. Popular among the higher-end custom builders.
Tapered - narrow, strongly tapered headstock (Kevin Gillies, Seagull, etc.).
Minstrel - similar to that of a minstrel banjo.
Crests:
Crown (Martin, standard Kanile'a, etc.)
Domed (Oscar Schmidt)
Open book (Epiphone)
Note that any of these designs can come in either symmetrical or asymmetrical varieties. In no way is this list comprehensive.