Fear is a disable that forces one or more units to flee towards their fountain, or towards their camps when applied on neutral creeps.
Fear forces the affected unit to drop any of its current order, and start moving towards their fountain instead. Fear applied on neutral creeps makes them retreat towards their camps. This means it fully cancels attack orders, ability cast orders and channeling spells. The feared unit only uses regular movement while fleeing, it does not utilize abilities. A feared unit is still subject to every other disable. The fear does not make them ignore movement imparing effects.
While affected by Fear, a unit cannot be given any orders, since Fear completely prevents acting. This means no orders can be queued up during it. Once the Fear expires, the unit stands still as if given a stop order. It still follows its auto-attack rules, based on the player's settings.
When a unit is affected by Taunts, Fear cannot make them flee, as the taunt takes priority in any case. However, since Fear completely prevents the unit from acting, it cannot attack the taunting unit, and therefore only follows it until able to attack again. The unit continues, or starts fleeing as soon as the taunt ends, if the Fear did not expire by then.
Fears are very similar to stuns, as they prevent the unit from doing almost anything. As such, it can be used to completely stop an enemy, protect allies, initiate and interrupt channeling spells by disrupting the enemy. Its advantage of stuns is that it can also be used to displace enemies. Depending on how long the fear lasts, this can be used to force enemies out of position.
Since fears force units to move, it can be used to break sources of invisibility which break upon moving, like Fade and Meld. It also combines very well with other movement based effects like Rupture and Mana Leak.
Sources of fear