Near-infrared light, which spans from 700 nanometers (nm) up to about 1000 nm, is beyond what the human eye can see, but most camera sensors can detect it and make use of it. During the day, the camera will deliver image like below,

a day and night camera uses an IR-cut filter. IR light is filtered out so that it does not distort the colors of images as the human eye sees them. The camera will deliver image like below,

Foscam IP camera built in IR-cut filter  with self-check function, when the camera is in night (black and white) mode, the IR-cut filter is removed, allowing the camera’s light sensitivity to reach down to 0.001 lux or lower.