Many of the data sets you will study will follow a similar distribution, with a peak around a certain value, and a few data points that lie outside the central cluster.  This curve is called the normal distribution, the bell curve or the Gaussian distribution. A typical normal distribution and its formula are shown below:

In this example, μ = 0 and σ = 1. As you can see, the population mean μ and population standard deviation σ appear explicitly in the formula for the normal curve.

The normal curve appears in many areas of science, due to a mathematical result called the Central Limit Theorem. This theorem states that when many distributions are added together, the sum will look like a normal distribution, no matter what the original distributions were. So if the quantity you are measuring is the result of many factors, as most biological processes are, that quantity will often be normally distributed.