Increased Attack Speed (IAS) is a coefficient in Diablo II that increases the speed of any melee attack and the Assassin's Martial Arts and Traps abilities. Most Increased Attack Speed caps off around 120-130%. Cast Rate is not affected.
Attack speed mechanics are closely related to frames; the better the increased attack speed, the fewer frames are required to perform an attack. The amount of increased attack speed required to remove one frame from an attack (commonly called "breakpoints) is integral to calculating attack speed; without reaching break points, nominal increases in attack speed do not actually improve attack speed in any way.