Affinity

An affinity is a bonus to skillgain in a given skill. Each player starts off with one affinity in a random skill or characteristic.

Additional affinities may be gained by killing other players in combat, provided the opponent is premium, has an affinity, and is of an enemy kingdom. It is possible to gain up to five affinities in each skill this way.

Skills for which you have an affinity can be seen by an asterisk symbol (*) next to their name in the skills window. Note that if you haven't unlocked that particular skill yet, it won't show up even if you have an affinity for it.

Source: http://wurmdev.blogspot.com/2008/09/weekend-update.html.