One of the confusing thing that many JC2s stumble upon is that variances in A-levels. This is partly attributed to the lack of understanding and introduction of the concept. By hypothesis testing, students should be exposed to three types of variances, $\sigma^2$, $s^2$ and the sample variance. So what is what?

$\sigma^2$ is our population variance. I like to teach my students that this is the TRUE value that no one knows, unless question states it.

$s^2$ is our unbiased estimate of population variance. One thing I constantly stress to students is that, $s^2\ne \sigma^2$. It is extremely important to identify that they are very different things. $s^2$ is an ESTIMATE of $\sigma^2$. And we get $s^2$ from the sample variance, which is the variance of the sample you collect.

I hope this helps to clarify their differences.