OS X Lion: About USB
Your computer has USB ports that let you easily connect external devices, such as a keyboard, mouse, trackpad, printer, mobile phone, iPod, or digital camera, without needing to restart your computer. The USB ports provide some power, so most devices don’t need their own power cords. You may be able to purchase adapters that let you connect non-USB devices to a USB port.
If you have a portable computer, USB devices can cause your computer battery to become depleted faster. If you are using the device for an extended period, it’s a good idea to connect the computer’s power adapter.
You can connect more than one device to a USB port with a USB hub. Connect the devices to the USB ports on the hub and then connect the hub to the computer. Most USB hubs have a power adapter and should be plugged into an outlet.