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.
To create effective minefields use Queue Command or Grid Building.
REMEMBER that mines use energy to remain cloaked! Trappers can also detonate enemy's mines using the attack command.
Trappers are stealthy and produce a tiny radar jamming field that enables them to lay mines while staying undercover.
A funny gimmick using Trappers can be done: Place 4 heavy mines on a location where you expect an enemy Commander to go (eg. near a metal spot) and detonate them simultaneously when he walks over them to give your enemy a big surprise.