Hi,
Yes, to be more specific, when no device is plugged into a USB port, the D+ and D- lines of the USB port will be in low state. If the device pluggedin is a full speed USB device(12Mbps), a resistor connected to Vcc line of the port inside the device will pull up the D+ line and in case of a slow speed USB device (1.5Mbps), the D- line will be pulled up. This change of DC voltage of the data line will be detected by the USB hub.
The PC software regularly keeps polling each connected hub to find out if any new device is connected to it and the hub in turn replies back when a device connection is detected. Then the PC software initiates its enumeration checks and finds out the hardware details of the connected device. It in turn looks for matching device driver software that best suits the connected hardware and allocates to it. If no suitable software is found, it flashes on screen a message as an 'unknown device found'.
Regards,
Laktronics