What are the differences between the difficulty levels in the Campaign and Skirmish modes?

Solution 1:

Distance an enemy unit must be within when the computer player unit looks for a new target:

easiest:    LOS (can be modified by sn-easiest-reaction-percentage)
easy:       LOS (can be modified by sn-easier-reaction-percentage)
moderate:   LOS * 2
hard:       LOS * 2
hardest:    LOS * 2

Computer players ignore relics on the easiest level.

If a non-exploring computer unit gets attacked, the computer player's attack delay is modified:

easiest:    allow attacking one minute ealier
easy:       allow attacking two minutes earlier
moderate:   allow attacking immediately
hard:       allow attacking immediately
moderate:   allow attacking immediately

After a wolf kills a unit, have it gorge itself (not attack again) for:

easiest:    35 seconds
easy:       30 seconds
moderate:   25 seconds
hard:       20 seconds
hardest:    15 seconds

Distance a unit must be within when a wolf looks for a new target:

easiest:    LOS * 0.5
easy:       LOS * 0.75
moderate:   LOS * 2
hard:       LOS * 2
hardest:    LOS * 2

Source: This Reddit /r/aoe2 post, which refers to the Official Computer Player Strategy Builder Guide (cpsb.doc) in the AOE 2 CD 'GOODIES' folder.

Solution 2:

may be it's too late but i think this question "does not take old"... AOE was criticized for the earliest versions because in the hard/hardest mode it does not take too much to beat the AI, so some cheating was introduced in "Age of the kings"(that's my own opinion). I can tell that because AI players can upgrade to feudal age since the first 5 mn easily(for any civilization) while human player can not do that with his resources(250 food !)[ less than the required 500 ].to prove that,start a new game ,in hardest mode ,and quit immediately ,then read the screen of the statistics ,you will find out that AI players have well higher scores because they were given more resources ! Another thing to mention is that AI managed units/buildings are some how "auto-healing" or "auto-reparing" and more efficient but it's not a bad thing,instead this makes the game more attractive and invites the human intelligence to improve each time and finest the strategy !