Measure vs. Computed Dimension

[PaulM1]

What is the difference between a Measure and a Computed Dimension? In what circumstances would I want to use one over the other?

[Alex_Hancock]

Measures are metrics aggregated across the selected data set, while computed dimensions apply logic to the data at each row level, and are then grouped at the grain of the look (which you could then calculate measures against). Typically you would use looker measures to create your metrics rather than providing those precalculated in the view sql.