A stealthy minelayer / minesweeper unit, that can make your base feel really safe. It produces light and heavy mines which are designed to eliminate different levels of threat. Light mines can stop Pawns, Grunts, Blitzes etc. and heavy can even stop most T2 units.
It's a cheap way to secure places where you don't have time to set up proper defenses, like your flanks.
Always lay mines away from your opponent's line of sight and avoid having your own units on the minefield.
Remember that mines use energy to remain cloaked!
Groundhogs can also detonate enemy's mines using the attack command. Groundhogs are stealthy and produce a tiny radar jamming field that enables them to lay mines while staying undercover.
A funny trick using Groundhogs: Lay 4 heavy mines in a place where you expect an enemy Commander to go (eg. near a metal spot) and detonate them simultaneously when he walks over them to make your enemy a big surprise.