A shepherd deals exclusively with sheep, while a herdsman workd with herds comprised of other animals (goats, cattle, ect.).
shep•herd (ˈʃɛp ərd)
n.1. a person who herds, tends, and guards sheep.2. a person who protects, guides, or watches over other people.3. a cleric or pastor.4.sheepdog.v.t.
5. to tend or guard as a shepherd.6. to watch over, guide, or lead.
herds•man (ˈhɜrdz mən)
n., pl. -men.1. the keeper of a herd, esp. of cattle or sheep.2. (cap.) the constellation Boötes.