Now let me talk bout how USB communicates with host(pc's etc)
USB has what you call endpoints,interfaces,configurations which are used to communicate with usb core.

The most basic form of USB communication is through something called an endpoint.
A USB endpoint can carry data in only one direction, either from the host computer to the device (called an OUT endpoint) or from the device to the host computer
(called an IN endpoint). Endpoints can be thought of as unidirectional pipes.
There are 4 types of endpoints which can be configured

1)CONTROL
Control endpoints are used to allow access to different parts of the USB device.
They are commonly used for configuring the device, retrieving information from/of device etc.These endpoints are usually small in size. Every USB device has a control endpoint called “endpoint 0”

2)INTERRUPT
Interrupt endpoints transfer small amounts of data at a fixed rate every time the
USB host asks the device for data. These endpoints are the primary transport
method for USB keyboards and mice.

3)BULK
Bulk endpoints transfer large amounts of data. These endpoints are usually
much larger (they can hold more characters at once)These endpoints are common on printers, storage and network devices.

4)ISOCHRONOUS
Isochronous endpoints also transfer large amounts of data, but the data is not
always guaranteed to make it through. These endpoints are used in devices that
can handle loss of data, and rely more on keepinga constant stream of data
flowing. Real-time data collections, such as audio and video devices, almost
always use these endpoints.