Day Vision refers to the operation of a camera under normal daylight conditions, whereas Night Vision refers to night time when the intensity of visible light is low such as 1Lux. 

When there is no visible light, the camera will have to rely on Infra-red (IR) for seeing.  During day time, IR is cut off.  During night time, IR is enabled.  During the transition period, the camera will depend on the visible light intensity to enable or disable IR. 

The above is called IR Filtering.  Another technique related to IR is artificial illumination such as by LED built in some cameras or externally by an illuminator.  This technique helps but is limited to a certain distance.