Arduino Uno R3
The R3 in the name stands for 'revision 3'. The Uno is one of the earliest models of Arduino prototyping board and it has been updated since its first release. The diagram below shows the main components.
When it is connected to a PC, the board takes power from the USB cable. If you want to run the board without being connected to a PC, you need to provide an external power supply. One of the simplest ways to do this is to use a 9V battery and a battery snap.
The board actually requires 5V, but the voltage regulator reduces the voltage supplied by the battery.
The digital and analogue pins can be configured as input or output according to the requirements of the application. For simple applications, sensors and actuators are connected directly to specific pins. One of the major limiting factors of any prototyping board is the number of pins available.
Arduino Web site
TutorialsPoint