Weapons skills change depending on which weapon a player has equipped. Slot skills are extra abilities that players can use in addition to weapon skills. Traits are special modifiers that can be purchased with trait points.
Skills are what players use in combat. They act very quickly in Guild Wars 2, and they each have their own unique advantages and disadvantages. Slot skills also give great modifiers and control abilities. Stun’s, snares, buffs, you name it, and they’re all in slot skills.