The Arduino board is built around an AVR microcontroller and it has all the required circuitary to get the built-in AVR microcontroller running. The Arduino can communicate with the other devices using its digital I/O, serial port, I2C port, SPI port etc. Any AVR microcontroller based board which follows the standard Arduino schematic and is flashed with the Arduino bootloader can be called an Arduino board. The Arduino IDE is so simple to use that anyone who has basic knowledge of c programming can quickly get started with it. The project on Getting started with Arduino explains about the steps required to get start with an Arduino board.
Fig. 4: External USB to TTL converter board for programming Arduino and serial communication