A USB system has an asymmetric design, consisting of a single host and multiple devices connected in a tree-like fashion using special hub devices. Up to 127 devices may be connected to a single host, but the count must include the hub devices as well, so the total useful number of connected devices diminishes somewhat.
The standard includes provision for power to the connected device. Some devices draw minimal power, so several may connect without needing extra power sources. Most hubs include power supplies which will power devices connected through them, but some devices draw enough that they need their own power. Powered hubs supply power to downstream devices (within prescribed limits) without draining power from the upstream connection.
The design of USB aimed to remove the need for adding separate cards into the computer's ISA or PCI bus, and improve plug-and-play capabilities by allowing devices to be hot swapped or added to the system without rebooting the computer. When the new device first plugs in, the host enumerates it and adds the software driver necessary to run it.
USB can connect peripherals such as mice, keyboards, scanners, digital cameras, printerss, hard drives, and networking components. For multimedia devices such as scanners and digital cameras, USB has become the standard connection method. For printers, USB has also grown in popularity and started displacing parallel ports because USB makes it simple to add more than one printer to a computer.
In the case of hard drives, USB seems unlikely to completely replace buses such as ATA (IDE) and SCSI because USB performs somewhat more slowly than those standards. The new Serial ATA standard allows transfer rates up to approximately 150 MB per second. However, USB has one important advantage in making it possible to install and remove devices without opening the computer case, making it useful for external hard disks. Today, a number of manufacturers offer portable USB hard drives that offer performance nearly indistinguishable from conventional ATA (IDE) drives.
USB has not completely replaced AT keyboard connections and PS/2 mouse connections, but virtually all PC motherboards today have one or more USB ports. As of 2003, most new motherboards have multiple USB 2.0 high-speed ports.
The USB 1.1 standard had two data rates: 1.5 Mbit/s for keyboards, mice, joysticks, and the like, and full speed at 12 Mbit/s. The major feature of the USB 2.0 standard is the addition of a Hi-Speed rate of 480 Mbit/s. It also clarifies minor technical errata. At its highest speed USB competes directly with FireWire (except in the areas of digital camcorders, USB has techonological limitations that prevent it from being viable in this area).
While USB defines four types of connectors for the attachment of devices to the bus, the mechanical layer has changed in some examples. For example, the IBM UltraPort is a proprietary USB connector located on the top of IBM's notebook LCDs. It uses a different mechanical connector while preserving the USB signaling and protocol.
An extension to USB called USB-On-The-Go allows a single port to act as either a host or a device - chosen by which end of the cable plugs into the socket on the unit. Even after the cable is hooked up and the units are talking, the two units may "swap" ends under program control. This facility targets units such as PDAs where the USB link might connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance.