Visible satellite images record visible light from the sun reflected back to the satellite by cloud tops, land surfaces and ocean surfaces. These black-and-white images show what your eye would see from space (if you were color blind) and hence represent nothing fancier than ordinary black-and-white photographs of the earth from space. The brightness of any feature on a visible satellite images depends on (1) how directly light from the sun strikes it, and (2) how reflective the feature is Cloud tops and snow and ice surfaces tend to reflect visible light best, so they tend to be the brightest (i.e., whitest) features on a visible satellite image. Ocean surfaces tend to reflect the least visible light, so they tend to be the darkest features.
Infrared satellite images record invisible infrared radiation emitted directly by cloud tops, land surfaces or ocean surfaces. The warmer an object is, the more intensely it emits radiation, so the intensity with which a feature on earth emits infrared radiation tells us about that feature’s temperature.