What's the difference between Modbus ASCII and Modbus RTU?

The original Modbus specification included two possible transmission modes: ASCII and RTU. Modbus RTU mode is the most common implementation, using binary coding and CRC error-checking. Modbus ASCII messages (though somewhat more readable because they use ASCII characters) is less efficient and uses less effective LRC error checking. ASCII mode uses ASCII characters to begin and end messages whereas RTU uses time gaps (3.5 character times) of silence for framing. The two modes are incompatible so a device configured for ASCII mode cannot communicate with one using RTU.  Modbus ASCII messages require twice as many bytes to transmit the same content as a Modbus RTU message.

For additional information, please see below the Modbus Protocol Specification document "PI-MBUS-300".