Primary skill
Primary skills are characteristics of heroes and creatures. There are four different primary skills: attack, defense, spell power and knowledge. Attack and defense can be categorized as might skills while power and knowledge are magic skills. Typically might heroes, like knights and rangers, advance more rapidly in might skills as for magic heroes, like necromancers and wizards, advance faster in magic skills. The advancement in primary skills vary considerably by the hero class on levels 1 to 9, but after that, hero's advancement generally becomes more balanced.
Attack skill
Attack skill together with defense are the main factors affecting the amount of damage creatures under hero's command deals. A hero's attack skill number is added to each of their creaturess attack rating, increasing the amount of damage they do in combat.
Defense skill
Defense skill together with attack are the main main factors affecting the amount of damage creatures under hero's command deals. A hero's defense skill is added to each of their army creature's defense rating, which decreases the amount of damage they take from enemy attacks.
It should be noted, that while defence is acceptable spelling, it is always spelled defense in Heroes of Might and Magic III (with the exception of Heroes Chronicles, where it is spelled defence).
Power skill
Power (or sometimes spell power) is the main factor for damage and duration of the spells. By default, all heroes start with at least one point in this skill. However, a map-maker can set the starting skill to 0. In that case, the hero will look and function as if he had power of one point. When the hero recieves the first point to power skill, he will still have only one point in power. After that, the points accumulate normally.
Knowledge skill
The knowledge skill determines the maximum amount of spell points a hero may have. It is calculated by multiplying knowledge with ten, so knowledge of 13 would give a hero a natural maximum of 130 spell points. With the secondary skill intelligence, this natural maximum can be increased up to a 100%, which will double the hero's maximum spell points. Additionally, with mana vortex or magic spring hero can temporarily double the spell points, which means that with expert intelligence, the maximum spell points will be quadrupled from original amount.
Like with power, all heroes typically start with at least one point in this skill. However, a mapmaker may set the starting skill to 0. In that case, the hero will look and function as if he had 1 point, but when he receives his first point, it will not have any effect.