14-bit RAW (NEF) file recording is available on certain Nikon DSLR cameras. The file size of images recorded at 14-bit is approximately 1.3 times larger than those recorded at 12-bit, however the extra data provides more latitude for post processing making a richer expression of tones possible and ultimately will provide superior quality output.
When evaluating the effect and comparing between 12-bit and 14-bit, the area that will show the most significant improvement between the two options is in the shadow detail. 14-bit will give you four times the information overall, but the shadow areas which receive much less exposure generally than mid tones or highlights, will have far greater detail.
A binary number with 12-bits of precision can record a number with 4096 different possible values (2 to the 12th power). 14-bit technology gives up to 16,384 possible values, four times as many compared to 12-bit. Increased bit rate does not result in more pixels or a wider range of colours, but rather more accurate precision on each pixel.
The frame rate at which images can be captured is slower with 14 bit as it takes the processor longer to write the increased volume of information to the memory card. So when shooting with the 14-bit setting activated, you need to evaluate how important your speed of shooting is in relation to the quality of the final image, as one will be affected by the other.