First mentioned in "I'll Be Seeing You, Part 1", the Hungry Heifer is originally a restaurant that Norm dislikes. He mentiones it as a new place downtown, but the food is bad and the service is bad as well. However, Norm keeps getting lured back by the specials, even though he knows repeated indigestion should have stopped him. He gradually comes to like the place more, and even defends it.
The Hungry Heifer is a place that sells cheap meat and meat-like substances (like "beff" and "loobster") and is apparantly designed for heavy eaters that aren't picky about what they eat.
We see the Hungry Heifer in Season 5's "Cheers: The Motion Picture" as a place Norm takes Woody. He is well known there by this time, getting the same "Norm!" greeting from everyone that he traditionally gets at Cheers. Corrine, a relief waitress at Cheers, also works here.
The Hungry Heifer is the center of the episode "Grease" from Season 9. This episode changed the backstory of the Heifer a bit; now, it's presented as a place Norm has known and loved since his college days. The owner, Sid Nelson, burned the place down in this episode to collect the insurance money.