As the name implies this is a library written for UART communication on a low-end ATtiny Microprocessor. I've created this 'rugged' version for the sake of simplicity... The library has got to be highly integrated into the program and it may be tricky to understand at first glance.
Here's a little explaination:
For the Transmitter of the UART library i got the source code and all relevant information from Marcel Meyer-Garcia (https://github.com/MarcelMG/AVR8_BitBang_UART_TX) If you find yourself trying to get a µC to communicate over UART but don't have a proper crystal for the timing of it all, this library might come handy. With the timer_comperator parameter of the UART_init() function, you can calibrate a hardware timer of the µC to form a Clock-Divider that provides the frequency for the desired baudrate. When you notice that you receive the wrong symbols on your Serial-Terminal you might consider tweaking the value of timer_compare a little that the counter times at the right pace.