A Cow run is opening a secret portal to the Secret Cow Level and killing all of the Hell Bovines and The Cow King. First, you will need to Kill Baal in the mode that you are wanting to do the Cow level in. By using Wirt's Leg found in the Stony Field and a Tome of Town Portal transmuted in the Horadric Cube, a red Portal will open up to The Secret Cow Level. Cow runs are used in a lot of ways. Some examples are for magic finding, finding Runes and Bases, or gaining experience.