A smiter is any character which uses skills from the Smiting Prayers skill line from the monk profession. Smiters are often monks, as they can increase the power of these skills using runes, but other professions, in particular Elementalists and Mesmers have been known to use a skill bar based predominantly on smiting prayers.
In a more vernacular sense, the description "smiter" usually applies to a character who maintains bonded enchantments, mainly Retribution, on another character, while taking a passive role out of aggro range of any foes encountered. The smiter is then able to deal damage to all foes that attack the target of his bonds-and to a lesser extent keep the target alive-without danger to himself. Most commonly, this role is used as the necessary counterpoint to a 600hp monk, combining smiting and protection bonds.
The most common skills used by monks of this type, which have become in a large part synonymous with smiting are: