There ought to be a "a brief history of time" book for computers and computer technology, leading up to today.
In a computer everything is run by software. The hardware exists to either execute the software, or allow different software to communicate with other software. Your mouse/trackpad/keyboard are running software on tiny processors inside them. They talk to the main CPU through a variety of communications channels and a number of sub-processors.
USB is one of these channels. It started out as a fairly low speed communications method meant mostly for human input such as mice and keyboards, and storage media such as Zip drives. It is one o father latest in a long line of slower communication channels.
Early computers had serial and parallel ports. Serial ports required between three and 25 wires to talk at speeds up to hundreds of thousands of bits per second. Parallel ports required between 10 and 36 wires, and over time were able to communicate up to tens of millions of bits per second. Faster, but they required a lot of wires, and so weren't suitable of objects, such as mice, that had to be cheap and moved frequently. Parallel ports were most frequently used for printers. Serial ports for mice and modems.
Apple introduced the apple desktop bus for their keyboards and mice, and IBM introduced the ps/2 port (personal computer 2) for their keyboards and mice. Parallel ports were still used for printers, scanners and since they were faster than serial for Zip disks and external drives. Serial ports were still used for modems since modems inherently talk in a serial manner and initially it was easier to develop cheaper modems that used that port, and modems never needed high speed.
USB was designed to replace all these uses. It needed to be fast enough for simple media storage, use few wires for thin cheap cables, and be cheap enough overall so you could still make $5 mice.
So that's what they made. It's essentially a smart serial protocol that runs on four wires, and initially ran at up to 12 million bits per second. It was upgraded to 480 million bits per second with USB 2.0, and recently USB 3.0 allows speeds up to 5 billion bits per second, although USB 3.0 requires more wires, or fiber optics cables, so it's a little more expensive.
When USB was released Apple released FireWire to compete. FireWire had a lot of advantages, it was faster for the first release, it could talk to more intelligent devices, and could even link computers without additional software. Unfortunately it also cost more, as it required more wires in cables, and more electronics on each end of the link. Apple also placed a few restrictions on it such that it was annoying to implement. It simply wasn't suitable for mice and keyboards, but was great for hard drives and networks. They both upgraded their speeds keeping pace with each other, but, you could say that FireWire lost.
Apple joined intel for the next generation of serial bus, and it's named thunderbolt. It's much faster, much more intelligent, and doesn't intend to replace USB, but to supply more speed and capability to those peripherals that need it.
Few peripherals need to right now, though. It's being used to display video in apple devices, and in some limited cases its being used for fast disk access. The latest version of USB is fast enough for most needs, though, and its cheaper so thunderbolt just doesn't have much traction in the market.
