What is difference between HMI and Monitor?

HMI / Human Machine Interface would imply two-way communication. A monitor only shows information to the user; the user cannot directly ask the monitor for information. an HMI will provide some way for the human to input data (such as a mouse). A monitor, with no way to input data would be more along the lines of an annunciator (old school ones were just a bank of lights for alarms).