A Combat style is the way you attack with your weapon. There are four different combat styles. Two offensive, one defensive, and one controlled combat style. The offensive will train either your Attack experience or your Strength experience. Defensive will train your Defense experience. And controlled will train all of your melee stats: Attack, Strength, Defense, and Hitpoints. Each damage you deal to an enemy you will recieve 9 experience in whatever combat style you are using (including hitpoints). However, if you are using controlled, it will share 3 experience for each skill.
Melee weapons vary in their attack styles. Some have four options, with either a controlled option or an alternative aggressive attack with a different action, while others have only three options. Spears have three alternate controlled options and a defensive option. Players intending to remain a pure with no defense need to take care when using the third option (alternative aggressive), as switching weapons may result in controlled or defensive becoming selected, often only spotted when an unwanted defence level is gained.