A very nice and simple explenation of what USART is and how it can be utilized with microcontrollers for robotics projects.
Again a very nice and simple explanation of the implementation UART functionality to your robotics project
This is a great tutorial on USART - serial communication for AVR microprocessors, contains well explained code and other users feedback. It is from the AVR Freaks website.
This page is an extension on the above tutorial, it covers interrupts