Spatial and Gray level resolution:

Sampling is the principal factor determining the spatial resolution of an image. Basically spatial resolution is the smallest discernible detail in an image.

As an example suppose we construct a chart with vertical lines of width W, and with space between the lines also having width W. A line-pair consists of one such line and its adjacent space. Thus width of line pair is and there are  line-pairs per unit distance. A widely used definition of resolution is simply the smallest number of discernible line pairs per unit distance; for es 100 line pairs/mm.

Gray level resolution: This refers to the smallest discernible change in gray level. The measurement of discernible changes in gray level is a highly subjective process.

We have considerable discretion regarding the number of Samples used to generate a digital image. But this is not true for the number of gray levels. Due to hardware constraints, the number of gray levels is usually an integer power of two. The most common value is 8 bits. It can vary depending on application. When an actual measure of physical resolution relating pixels and level of detail they resolve in the original scene are not necessary, it is not uncommon to refer to an L-level digital image of size as having a spatial resolution of  pixels and a gray level resolution of L levels.