The USB (Universal Serial Bus) is a high speed bus on an external cable that plugs into a PC. The bus has its own communication protocols and doesn't use any IRQs, I/0 addresses (or other bus-resources) on the external bus cables. Communication is by packets something like the Internet, only there are time-slice allocations which prevent any one device from hogging the bus if other devices need it. There are free time slots which allow every device to send a short message to the bus controller without any need for IRQs.
However, the USB bus controller inside the PC does have an IRQ and an address on the PCI bus (or ISA) which are used for communication between the CPU and all devices on the USB. Thus there's no resource allocations needed for the individual devices on the USB.
The devices on the USB do have IDs, just like cards do on the PCI bus. So Linux likely maintains a table of these IDs so that device drivers can check them to find their device. The USB also support "hot plug".
"Hot plug" is where you plug something into a PC (usually via a cable connection) and it is instantly detected. If required, it is configured with bus-resources. The driver for it is also started, perhaps by loading a module for it. For this to work the hardware used must be designed for hot plugging. One can hot plug certain PCI cards (Cardbus), USB devices, and IEEE 1394 devices (Firewire).
When a new device is detected, it's registers are read so as to get the ID number of the device. Then to find a driver, Linux must maintain a table mapping ID numbers to drivers. It wasn't until kernel 2.4 that such a table existed as Linux shunned centralized PnP.
External devices that connect to the serial port via a cable (such as external modems) can also be called Plug-and-Play. Since only the serial port itself needs bus-resources (an IRQ and I/O address) there are no bus-resources to allocate to such plug-in devices. In this case, PnP is used only to identify the modem (read it's model code number). This could be important if the modem is a software modem (linmodem) and requires a special driver. There is a special PnP specification for such external serial devices (something connected to the serial port).
Linux doesn't support this yet ?? For a hardware modem, the ordinary serial driver will work OK so there's little need for using the special serial PnP to find a driver. You still need to tell the communications program what port (such as /dev/ttyS1) the modem is on. With PnP you wouldn't need to even do this. With the advent of software modems that have Linux drivers (linmodems), it would be nice to have the appropriate driver install itself automatically via PnP.