Cumulative probability functions
A cumulative probability function (the cdf) gives the probability that a value from a distribution will be less than or equal to the "input" value. This is also the area "to the left" under the density curve. Therefore it climbs from 0 to 1. (Calculus: It's the integral of the density function from negative infinity to the value in question.) Its name in Fathom is <something>Cumulative, for example, TCumulative(x, df), which is the cumulative t distribution for df degrees of freedom. On some calculators, it's called T_CDF.
Its inverse is a quantile function.