Choice Scarf: Boosts your speed stat by 1.5, but at the cost of being only able to use one move until you switch.
Choice Specs: Boosts your Sp. Attack stat by 1.5, but at the cost of being only able to use one move 'till you switch.
Choice Band: Boosts your Attack stat by 1.5, but at the cost of being only able to use one move 'till you switch.
Basically, they give you great power/speed, but limit you to only one move.
Also, this should have been in the simple answer simple question thread.