Universal Serial Bus

USB: USB stands for Universal Serial Bus.USB started in late 1990's( around 1996). USB is a hardware interface with defined protocols that enable Host computer to connect to different peripheral devices attached . It connects to a variety of devices like keyboard, mouse, printer, camera, pendrive, external HDD, etc. Hence for many of these kind of devices, USB has become common to have.
Design for USB is standardized by USB Implementers Forum or USB-IF. Currently one Host controller can support at max of 127 device at a time.
Initially when USB was started it was called as 1.0 with data rate of 12 Mbit/s but currently the USB we are using is USB 2.0 with data rate of 480 Mbit/sec. USB 2.0 was started in 2000
Features:

• USB provides Hot plugging and Hot swapping facility, which means you can add or remove peripherals without rebooting your PC or computer(Host controller).
• USB can also supply power to some of the devices which draw small amount of power like Pen Drive, mouse, keyboard.
• speed upto 480 Mbits/sec
• USB 2.0 can have max length of cable up to 5 meters and maximum hubs in series can be only 5.
• Data Cable for USB 2.0 use twisted pair to reduce noise and crosstalk.
• The USB 2.0 specifications provide a 5 V supply on a single wire from which connected USB devices may draw power. 
• Devices may be low powered, high powered, or self powered.

 Now a little bit of deep insight into USB.
To get USB working, you need Hardware and Software:
USb Hardware:                                     

HOST controller/Root HUB         
HUB
Device

USB Software:
Device Drivers
Drivers
Controller driver

Physical connection occurs between HOST controller and client bus interface( root hub) USB Host controllers are of following type:
Universal Host Controller Interface(UHCI)
Open Host Controller Interface(OHCI)
Enhanced Host Controller(EHCI)

USB Devices are of three types:
High speed:High speed devices see only high speed transactions, full or low speed devices are accessed via high speed split transactions.

Full Speed: Maximum data rate of 12 Mb/s
Low speed: These are limited in throughput(1.5 Mb/s) and also in feature.

USB connectors are of two types: Type A and type B
Type "A" connector supplies power and "B" receives power. Hence protecting users from connecting same type of connectors, same type of connectors can never be connected. Generally Type "A" connectors are used Host and Type B is used in devices.