This difficulty is designed for players who know the basics of WarCraft III but do not have much experience in playing it. Also select this difficulty, if you want to quickly follow the story rather than take time to beat some challenging encounters.
This difficulty is designed for players who are somewhat experienced with WarCraft III but have little experience in tough melee games. If you can beat a normal melee computer, and you have played through the Blizzard campaign (without cheats), this is probably the right difficulty for you.
This difficulty is designed for players who regularly defeat insane melee computers and have honed their skills and combat tactics in melee games against other human players. If you select this difficulty, be ready to face challenging encounters.
This difficulty is designed for players, who can beat an insane melee computer blindfolded and don't usually play custom maps, because they think that "people always make their scenarios too easy."