Confused is a status most commonly caused by a confusion gas trap or an ill advised potion of confusion. When confused, you probably won't move in the direction of your choosing; instead, you will move in a random direction. However, the experienced rogue will choose to stand still... or use items in his inventory, for even while confused, it's possible to aim and throw items (ie wands, staffs, potions, darts) with 100% accuracy.
Confused enemies are not so smart. They will not stand still, always moving, moving, moving. This means that they won't cast spells (pretty sure about that), which is huge. Tossing a potion of confusion is even more excellent when enemies are anywhere near lava, fire, brimstone, traps, or a chasm. But wait, there's more. Confused enemies will also attack one another as they bump into one another as random movements dictate. So throwing a potion of confusion into a tightly packed group is a good way to lessen their numbers and weaken any survivors. Psst, because of their low HP, troublesome spellcasters will likely be the first to get 'bumped' off by their beefy companions.
Confusion gas is flammable, too. So if you're feeling a bit sadistic, you can always light 'em up before the gas dissipates. They'll still be confused, they'll just be burning also.
A runic Weapon of Confusion will also impart a confused state upon its victim, but it's not quite as much fun.
Monsters that Flit move as though confused 33% of the time, except they will not attack allies or enemies when flitting, only make movements.