Within group variance and between group variance

What is the difference between WITHIN GROUP VARIANCE and BETWEEN GROUP VARIANCE?
a. With group variance estimates the population variance using all the data values, and between group variance estimates the population variance using the means
b.  Within group variance estimates the population variance using the means, and between group variance estimates the population variance using all the data values.
c.  Within group variance relies on relationships in the data set and the between group variance relies on relationships i different settings
d.  They actually have the same meaning