Thermal imaging is a way to remotely sense temperature, relative to its environment. Thermal imaging cameras sense radiation in the form of infrared light. This information is typically displayed on an LCD screen, where different colour ranges identify the changes in temperature. Thermal imaging cameras range from handheld devices that fit in your pocket, to handheld cameras for basic thermography up to high-end cameras capable of measuring high temperature ranges and clearer high resolution images.
Read more on thermal imaging here, as well as applications.