A bus is a shared communication pathway that connects devices to a computer and defines how they are discovered, addressed and exchange data. USB and PCI Express are two of the most familiar examples.
In practice
Your computer's USB controller manages a bus to which hubs, keyboards and drives attach. The USB bus driver in Windows discovers each device, reads its identifiers and passes them to Plug and Play so a suitable function driver can be loaded.
Common confusion
Bus refers to the pathway and its rules, not the physical cable alone. A USB-C connector can carry USB data, display signals and power at once, which involves more than one bus or protocol. Bus also differs from bandwidth, which is how much data the bus can carry.
Why it matters to readers
Problems that appear across many devices at once often point to the bus they share. Viewing devices by connection in Device Manager reveals which bus each device sits on.
A closer look
Buses nest inside one another. A USB host controller is itself a device on the PCI Express bus, and a USB hub creates further branches. Device Manager's view by connection reveals this tree, showing that a webcam may sit beneath a hub, which sits beneath a controller, which sits beneath the PCI Express root. Each level has its own driver.
