Both feed dynamic values into your data, but they live in different places and do different jobs.
Tip: Use a variable when you want a cell in your sheet to control the data. Use a parameter when you want to tweak how an Astrato measure calculates, just for one formula.
Note: When both point at the same thing: if a variable is mapped to the very same measure parameter that you also override in a formula, the formula’s parameter wins. You’ll rarely hit this, but that’s the tie-breaker.