Abrasion takes place when stones become embedded in the glacier, as the glacier moves these stones slowly scratch along the surface of the ground below. Abrasian acts similarly to sandpaper, leaving a fairly smooth surface to the rock it has worn down.
Plucking takes place when the glacier has been fairly stationary for a while, so patches of rock from the ground below have become frozen to the glacier. When the glacier then moves, this patch of ground is forced out of the ground as it is attached to the glacier itself. As you can probably imagine, this leaves behind a much more jagged, uneven surface.
