Why is it correct to say a normal distribution and the standard normal distribution?

A normal distribution refers to any bell-shaped distribution characterized by its mean and standard deviation, allowing for a variety of shapes depending on these parameters. The standard normal distribution, however, is a specific case of a normal distribution where the mean is 0 and the standard deviation is 1. This standardization allows for easier comparison and calculation of probabilities using z-scores, which represent the number of standard deviations a data point is from the mean. Thus, while all standard normal distributions are normal distributions, not all normal distributions are standard normal distributions.