Practically, the main interface currently used for peripheral equipment.
First version appeared in 1995, and technology was promptly adapted by manufacturers. Ability to hot-plug the devices without the need for computer restart and easy port multiplication (up to 127 devices) by using hubs were a big advantage over previous standards.
Uses typically 4-wire cabling, with +5 volt DC power (up to 500 mA) provided to devices, and +3.3 volt logical levels for serial transmission of data. Latest USB 3.1 version have extended Power Delivery capability, providing voltage up to 20 V and current up to 5 A for compatible devices and connectors.
Good description of various technical details:
USB in a NutShell
Nice collection of USB identificators assigned to the manufacturers/vendors and particular devices:
The USB ID Repository
Vendor ID and Product ID are very useful to identify the device (and find appropriate driver), even if it does not have correct 'human-readable' marking. Handy utilities can help to quickly browse USB information reported by device, for example USBDeview by Nir Sofer:
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