What is the difference between a measure and a calculated column?

A calculated column is computed at refresh, row by row, and stored in the model — it has row context and consumes memory. Use one when you need a value to slice, filter or group by.

A measure is computed at query time inside the filter context created by the visual. It stores nothing and must aggregate. Use one for anything that goes in the Values well.

Default to measures; a calculated column that only feeds an aggregation is usually better written as SUMX or moved into Power Query.