A Z-score describes the position of a point in terms of its distance from the mean when it is measured in the standard deviation units. The z-score is always positive if the value of z-score lies above the mean and it is negative if its value is below the mean.
Z-score is also known as standard score as it allows comparison of scores on different types of variables by standardizing the distribution.
A standard normal distribution is a normally shaped distribution with a mean of value as 0 and a standard deviation of value as 1.