When the PC host senses that your USB peripheral device has been connected, the host sort of "taps the device in the shoulder" and asks the device to describe itself.

The peripheral device then provides all sorts of information about itself: who made it, how its Configurations, Interfaces, and Endpoints are organized (see the following slides if you're not familiar with these terms), and other information about the type of device it is and the amount and nature of the data that the device expects to send and receive.

The device provides this information to the PC in the form of tables of numbers called Descriptors.

The PC Host looks at this information very carefully. If it thinks the device is okay, then the Host will locate and run whatever device drivers may be needed for this device, then start exchanging data with the device as needed.

On the other hand, if the Host does NOT like what it sees, it will ignore the device and never start it up. Depending on the operating system version, it may give the user some indication that the device has a problem, but it won't tell what specifically is wrong.