USB 3.0 has a bandwidth of up to 5 Gbps. That makes it up to 10 times faster than USB 2.0. With this hub you can use the fast USB 3.0 for four devices.
USB 3.0 is downwardly compatible, it fits also USB 2.0 cables and devices (with their transfer rates). The hub is bus powered. There is a 5 V DC jack on the case. If you want to run more external HDDs you can buy a 5 V power supply to get enough power.