In a box and whiskers plot, what values do the box boundaries, line, and whiskers represent?

The box always extends from the 25th to 75th percentiles. These limits are sometimes called the hinges of the plot. Surprisingly, there are multiple ways to compute the percentile values. This page explains the method Prism uses. 

The line in the middle of the box is plotted at the median. You can not choose a different value, but Prism also lets you put a "+" at the mean. 

There are multiple ways to define the whiskers, and Prism 5 gives you a choice. Prism 4 always plots the whiskers down to the minimum, and up to the maximum, values.

Prism is designed to create the box-and-whiskers plot from raw data as detailed here. If you want to directly enter the five values plotted on a box-and-whiskers graph, use this workaround.