confusion sampling vs quantization?
Sampling is the process of making the x-axis (time) discrete and quantization is the process of making the y-axis (magnitude) discrete.  You can sample without quantization (such as done with an analog sample and hold circuit). Quantization is introduced through rounding or truncation when the sampled analog signal is mapped to a digital representation. 



Sampling only causes aliasing distortion to the signal; if there is no energy above the half the sampling rate for real signals with real sampling, or above the sampling rate for complex signals with complex sampling, absolutely no distortion occurs assuming a perfect sampling clock with no hitter/phase noise (both aspects are not physically possible to totally eliminate but can each be reduced to below our level of concern).

Many A/D's, such as successive approximation converters, are implemented with a fast analog sample and hold so that the sampled signal is held while the conversion to digital takes place.  

Quantization noise adds noise to the resulting signal, well approximated in most conditions as a uniform white noise: the noise distribution in magnitude is uniformly distributed over the magnitude of a quantization level and is white in frequency to the extent that each sample of the noise is independent of the next.

From that approximation of the noise we can arrive at the SNR estimate for a full scale sine wave to be 6.02 dB/bit + 1.76 dB, which we can increase further by “over sampling”; since the noise added is white, we can filter out the noise in the band we don’t need.

I have detailed that process further at this post: What are advantages of having higher sampling rate of a signal?