Affinity

An affinity gives a 10% skill gain bonus to a given skill (or characteristic) and is marked by an '*' behind the skill's name in the skill list. Each player starts with one permanent affinity in a random skill.

It is possible to earn additional affinities by participating in unique slayings. The chance of obtaining an affinity during a slaying decreases depending on the number of affinities you already have. It is possible to have up to five affinities.

It is also possible to obtain temporary affinties via cooking.

More info
http://www.wurmpedia.com/index.php/Affinity