Also the USART automatically senses the start of transmission of RX line and then inputs the whole byte and when it has the byte it informs you(CPU) to read that data from one of its registers. The USART of AVR is very versatile and can be setup

Template library for use USART on AVR

This the USART library that can be used on AVR micro controller, it has functions for sending string and receiving strings-This is the USART library that can be used on AVR micro controller, it has functions for sending string and receiving strings

USART Library for C++ Compiler AVR MCU