What is Cisco QSFP to QSFP Copper Direct-Attach Cables ?

Cisco QSFP to QSFP copper direct-attach 40GBASE-CR4 cables are suitable for very short distances and offer a very cost-effective way to establish a 40-gigabit link between QSFP ports of Cisco switches within racks and across adjacent racks. Cisco currently offers passive cables in lengths of 0.5, 1, 2, 3, 4 and 5 meters and active cables in lengths of 7 and 10 meters.