Background
USB was developed in the early 1990's by industry leaders such as: Compaq, Intel, Microsoft, and NEC. The universal serial bus (USB) specification was developed as an external expansion bus standard. The goals of the standard included: ease of use, device interoperability, and low cost to name a few. 

The ease of use is probably the most important feature of the standard. Consumers wanted the ability to plug in devices at will and start using them without having to load software or reboot their computers. USB can also supply power to the devices, this makes the devices less complex and cheaper to install.

All USB devices plugged into the bus are supposed to interact with all other devices on the bus. By minimizing device conflicts, the user will have a more enjoyable time on their computer.

Another primary goal of USB is low cost. One way that was achieved was settling on a serial transfer instead of parallel. A cable for serial transfer is cheaper to manufacture than parallel. Another way was supplying power to the USB device. This reduced the cost by saving on external power supplies.

This is an over simplification of the USB standard and goals, but it shows the robustness and direction of USB.

Architecture
The USB bus allows devices to be attached, configured, used, and detached while the host and other devices are still operating. The attached devices share the 12Mbs bus that the host schedules. The physical topology is a tiered-star configuration with a hub as the center of each star. Each wire connection is point-to-point between a host to a hub or function, or a hub to a hub or function. (Let me define a few terms: A USB function is a device that provides some capability to the computer. A USB hub is a device that connects other hub and functions to a host. A USB device is a hub or function generically.)
             
There is only one host in a USB system. There can be no more than 3 hubs chained between a function and a host.
             
USB transfers signal and power across a 4-conductor cable, with signaling across 2-conductors and power across 2-conductors. There are 2 data rates, currently, at which a device can operate. They are 12Mbs and 1.5Mbs. A device can also draw up to 500mA of power through the cable.