Excel Formulas and Functions
This is what Excel is all about, being able to do calculations. From simple to complex calculations you can do this in Excel. It really comes down to what you want to do. You do this with formulas.
Functions are a whole lot of calculations, statistics, logical and so much more that you can use on their own or along with formulas to achieve what you need.
You should start with formulas and then find the functions that are useful to you.