To reach the Secret Cow Level, you must first have beaten Diablo. Once you've done this return to the Rogue Encampment.
Using the Horadric Cube, obtained in Act II, you will place a Tome of Town Portal and Wirt's Leg (obtained in Tristram), before selecting Transmute.
This will open a red portal within the Rogue Encampment that will take you to the cow level.
Within, you will find hundreds of vicious... cows... Armed with polearms. It's an excellent place to grind experience, especially if you're playing on a server. I was able to take my paladin from 50 to 99 in only a handful of runs. It's worth noting that if you kill the Cow King, you will no longer be able to create the portal. You can, however, still enter the portal if you're playing on a server in which someone else has created said portal.