The UNO is arguably the most popular Arduino. It is powered by an Atmega328 processor operating at 16MHz, includes 32KB of program memory, 1KB of EEPROM, 2KB of RAM, has 14 digital I/O, 6 analog inputs, and both 5V and 3.3V power rails.
A power jack is included on the UNO, allowing it to be powered by an external power adapter There is also a VIN option available for connecting the UNO to batteries.
The physical dimensions of the UNO (69mm x 54mm) make it a small development board that can easily fit into many projects and the four screw holes allow designers to securely fit them into place. (Adapted from MakerPro)