EMZETT.USB
Image: Mobius at en.wikipedia, Public domain, Wikimedia Commons
In short: “Universal Serial Bus” — the universal standard for connecting peripherals (mouse, keyboard, storage, printer and many more) to a computer, including power supply.
In more detail: USB covers both data transfer and power supply at once, which makes separate power adapters unnecessary for many small devices. There are various connector shapes (USB-A, USB-B, USB-C) and speed generations (e.g. USB 2.0, 3.0, 3.2) — connector type and speed are independent properties.
In Depth
History and basic idea
Before USB (introduced in 1996), practically every device class needed its own connector type and its own drivers — mouse (PS/2), keyboard (PS/2 or DIN), printer (parallel interface) and modem (serial interface) all had different, incompatible ports, often with bulky screw connectors. USB unified this via a shared electrical and logical protocol: devices announce themselves to the operating system when plugged in (“plug and play”) and are automatically recognised, with no restart or manual driver installation needed for most standard devices — a revolutionary convenience gain at the time.
Speed generations in detail
Speed generations have multiplied several times since then: USB 1.1 (1998, 12 Mbit/s), USB 2.0 (2000, 480 Mbit/s, still completely sufficient today for many simple peripherals like keyboards and mice, since these only send a few KB/s of data anyway), USB 3.0/3.1/3.2 (from 2008, 5, 10 or 20 Gbit/s depending on the exact sub-variant — the naming of these intermediate steps was notoriously confusing and was renamed several times by the USB Forum), and the newest USB4 (from 2019, up to 40 Gbit/s, technically closely related to and partly compatible with Thunderbolt 3/4). Important here: the speed generation is independent of the physical connector shape — a USB-C connector can carry both slow USB 2.0 and super-fast USB4, depending on what the cable and devices actually support. A cheap USB-C charging cable often only supports USB 2.0 speed, even though the connector looks identical to a full-fledged USB4 cable — a common source of frustration (“why is my external SSD so slow?”).
Power supply
Besides data, USB also transmits power, which makes separate power adapters unnecessary for many small devices. There’s been massive development here too: USB 2.0 delivered only 2.5 watts by default (500 mA at 5V), USB 3.0 somewhat more (4.5 watts), while “USB Power Delivery” (USB-PD, usually via USB-C connector) can now deliver up to 240 watts — enough to even charge powerful laptops, for which a manufacturer-specific power adapter used to always be needed. This unification of charging standards was also a central driver behind EU regulation, which mandates USB-C as the required charging port for most small electronic devices.
USB hubs and bandwidth sharing
A USB hub lets several devices run off a single USB port — however, all connected devices share the available bandwidth of the hub’s port. For pure input devices (mouse, keyboard), this practically doesn’t matter, but with several high-speed devices active at once (e.g. several external SSDs), it can become a noticeable bottleneck.