The t distribution is a family of distributions that are similar to the z distribution, in that they are all symmetric and bell-shaped around zero with asymptotic tails.

However, the t distribution has broader tails than does the z distribution. Each t distribution is identified by a parameter known as the degrees of freedom df. The d f determines the extent of broadness— the smaller the df, the broader the tails. Since the t distribution is defined by the degrees of freedom, it is common to refer to it as the redistribution. P-69