Infrared Cameras are thermal cameras that are primarily used for predictive maintenance, energy audits and scientific imaging. They typically include temperature measurement with the image and have analysis software packages.
Thermal Imagers are thermal cameras that are primarily used for surveillance and fire fighting applications. These include handheld imagers, head mounted imagers, vehicle mounted imagers and thermal weapon sights.
Night Vision devices are those units that enable you to see in the dark by amplifying the limited available ambient light in the scene. These units have the typical "green" night vision image and can be handheld, head mounted or weapon sights.