It reduces movement and attack speed.
Standard cold effect will only last for a while, and only to the monster hit by you. The aura affects everyone within a radius, and also adds a bunch of cold damage to your attack to sweeten it up even more.
also. even cold immune monsters are slowed down by this aura, making it useful against cold immunes if you want to slow them down, but i think the cold damage still doesnt affect them, making it more effective on a merc in the runeword doom than as your main aura.