TTL (transistor-transistor logic) voltage is 5V for HIGH and 0V for LOW. CMOS (complementary metal oxide semiconductor) voltage is 0V for LOW and 3V for HIGH.
UART means Universal asynchronours reciever transmitter
and USART means Universal synchronous asynchronous reciever transmitter
yes indeed the CD series can be operated over a huge range of voltages, 3V to 15V if i remember. i really like the idea that u can operate it from a 9V batt without the need for a 5V reg.