Uno R3 is a microcontroller board based on the ATmega328 (datasheet), fully compatible with ARDUINO UNO REV3.
It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz quartz crystal, a USB connection, a power jack, 2 ICSP headers, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.
Note that the two ICSP headers are separately used to program the firmware to ATMEGA16U2-MU and ATMEGA328P-PU, but generally, the two chips have been programmed well.
The Uno R3 differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features the Atmega16U2 programmed as a USB-to-serial converter.
The UNO is the best board for getting started with electronics and coding. If this is your first experience tinkering with the platform, the UNO is the most robust board you can start playing with.