USB is Universal Serial Bus. If your question relates to a specific chip, please mention it in your question.
The Universal Serial Bus (USB) is a 4-wire bus that supports communication between a host and one or more (up to 127) peripherals. The host controller allocates the USB bandwidth to attached devices through a token-based protocol. The bus supports hot-plugging and dynamic configuration of the devices. All transactions are initiated by the host controller. (source)
The standard specifies the connectors, cables and protocols used on the bus.
USB has different speed classes: Low Speed (1.5 Mbps), Full Speed (12 Mbps), High Speed (480 Mbps) and Super Speed (up to 5 Gbps). Low speed and full speed have existed from the initial versions of the specification, while high speed was introduced with USB 2.0 and super speed was introduced with USB 3.0.
The specification has undergone several major revisions since it's official release in 1995, the latest being USB 3.0.
Logically, USB implements a tree-structured master-slave bus. However, each physical connection is point-to-point between a host or upstream port and a device or downstream port.