Boxing
Defaults: None.
This is the skill of trained punching. Roll against Boxing to hit with a punch. Boxing does not improve kicking ability – use Brawling or Karate for that. Boxing improves damage: if you know Boxing at DX+1 level, add +1 per die to basic thrust damage when you calculate punching damage. Add +2 per die if you know Boxing at DX+2 or better! Work out damage ahead of time and record it on your character sheet.
When you defend with bare hands, Boxing allows you to parry two different attacks per turn, one with each hand. Your Parry score is (skill/2) + 3, rounded down. Boxing parries are at -2 vs. kicks and -3 vs. weapons other than thrusting attacks. Boxing also gives an improved retreating bonus when you parry; see Retreat. For more on barehanded parries, see Parrying Unarmed.
Martial Arts
This skill isn't specifically associated with the Boxing style. Any fighting style that teaches a "scientific" approach to punching – but not other unarmed strikes – might include it.
There's rarely a reason to learn Boxing if you know Karate. However, encumbrance penalizes attacks and parries with Karate but not with Boxing. Karate-users who believe that they could end up fighting unarmed while clad in heavy armor might therefore wish to invest in Boxing as well (but see Armor Familiarity).
The damage bonus for Boxing only applies when attacking with that skill. It doesn't "stack" with Brawling or Karate bonuses.