merge helps us add columns (variables) to the existing dataset. Intuitively, this means that we have data related to different variables on the same set of people (or any observed unit, such as district, country, etc.) stored in different files; and we want to combine these together -> this takes the form of stacking data side by side.

The dataset already loaded into Stata is called the master dataset while the one to be appended is called the using dataset.

The syntax is slightly more involved than for append.