This blurb from the WOW Wiki - Calculations and Formulas. http://www.wowwiki.com/
Each point of weapon skill you have over your opponent's increases your chances to crit and hit by 0.04%, and reduces your opponent's chance to block, parry, and dodge by 0.04%. Effectively you can consider it to be the inverse of the defense skill.
In addition, against mobs that are two or more levels above you, you will have a chance to land a glancing blow instead of a normal hit. A glancing blow may not crit and has a damage reduction proportional to the weapon skill to defense difference. Weapon skill reduces the severity of the damage reduction, not your chance to land a glancing blow.
For example, against mobs three levels above your own, with 300 weapon skill approximately 40% of your swings will be glancing blows, and the damage for those will on average be reduced by 30%. With +5 weapon skill, you will see the same amount of glancing blows (40%), but only 15% damage reduction. With +10 weapon skill, the damage reduction becomes negligible.
Each point of weapon skill you have over your opponent's increases your chances to crit and hit by 0.04%, and reduces your opponent's chance to block, parry, and dodge by 0.04%. Effectively you can consider it to be the inverse of the defense skill.
In addition, against mobs that are two or more levels above you, you will have a chance to land a glancing blow instead of a normal hit. A glancing blow may not crit and has a damage reduction proportional to the weapon skill to defense difference. Weapon skill reduces the severity of the damage reduction, not your chance to land a glancing blow.
For example, against mobs three levels above your own, with 300 weapon skill approximately 40% of your swings will be glancing blows, and the damage for those will on average be reduced by 30%. With +5 weapon skill, you will see the same amount of glancing blows (40%), but only 15% damage reduction. With +10 weapon skill, the damage reduction becomes negligible.