Invisibility plays a bigger role in Warcraft III than it did in Warcraft II however unlike Warcraft II, there are ways to detect Invisibility. Invisibility is typically used to hide, scout or to launch sneak attacks.

Invisibility is NOT the cloaking of StarCraft. In StarCraft, cloaking allows units to attack while they are invisible and creates a blur effect giving it away to players that notice it. In Warcraft III, Invisible units will reveal themselves if they do anything but move or stop. Invisible units are totally invisible unless someone has a way to detect invisibility.

Typically many players ignore Invisibility detection altogether. But Invisible Shades can really determine the outcome of a game as they find defenseless expansions and track the enemy army. They should not be ignored.

Invisibility Options

Hero Items
Invisibility Potions - You can buy invisibility potions at the Goblin Merchant.
Shadowmeld - There is an item that allows Heroes to gain the Shadowmeld ability.
Humans
Sorceress - Invisibility. Humans are the kings of Invisibility because they can make any unit invisible using the Sorceress.
Orcs
Blademaster - Wind Walk. Blademasters can use Wind Walk to launch sneak attacks, to spy, or to get away.
Undead
Carrion Beetles/Crypt Fiends - Burrow
Shades - Shades are permanently invisible ground based Observers (of StarCraft's Protoss) with no attack. Typically players place them in an enemy town to spy on them.
Night Elves
Shadow Meld. The Archer, Huntress, Warden, and Priestess of the Moon are invisible at night when they are not moving.
Invisibility Detection
Remember: just because you can't see them, it doesn't mean you can't hit them! If you know invisible or shadowmelded units are around, use an area attack spell like Blizzard, Starfall, Flame Strike, or Rain of Fire. You can also use attack ground with siege units like the Demolisher.

Hero Items
Dust of Appereance - Purchased at player-built shops
Crystal Ball - Reveals a targeted area. Invisible units are also revealed by the Crystal Ball's effect.
Sentry Wards - Drop Sentry Wards to spy on an area, and reveal invisible units.
Humans
Flying Machines
Mortar Teams - Flare
Towers - Magical Sentry. This is an upgrade at the Arcane Sanctum to allow you to see invisible units using your Towers. The Humans have the easiest Invisibility detection.
Orcs
Far Seer - Far Sight
Witch Doctor - Sentry Ward
Undead
Shades - Shades see invisible units.
Night Elves

Huntress - Sentinel
Priestess of the Moon - Scout
Neutral

Beastmaster - Hawk