Hello all, I am taking a probability and statistics class and I've entered it late, like 3 weeks in.

We did some area under curves, and I saw the use of x-bar (mean, an x with a bar above it) and s (standard deviation). Now I am seeing the Greek sign Mu (μ) for mean, and Sigma (σ) for standard deviation. What is the difference between these two uses? I am confused because I think there are formulas that apply to the area under a normal curve and others that apply to any curve? Can anyone clarify for me? Thanks.