Geoprocessing is what you use for many day-to-day GIS tasks. Geoprocessing tools perform small yet essential operations on geographic data, such as extracting and overlaying data, changing map projections, adding a column to a table, calculating attribute values, polygon overlays, and calculating optimal routes, to name a few.
There are hundreds of tools at your disposal that you can chain together using ModelBuilder and Python. This illustration shows a model where the input feature class is clipped to a study area and then transformed into another map projection.