They are Attack, Defense, Power and Knowledge. Knowledge has to do with the maximum number of spell casting points a particular hero can have. A certain amount of spell points are replinished every day and there are means of replinishing the max spell points sooner. There are many spells that can be cast with 5 different levels of spells. Anyone who plays fantasy RPGs will be quite familiar with the magic spells and their effects.
Heroes also obtain secondary skills.
Just to name a few, 'Eagle Eye' is a skill which gives your hero a chance to learn a spell the opposing hero casts during combat. Leadership' will raise the moral rating of a hero's troops giving them a greater possibility to attack twice in one combat round. There are 28 secondary skills. If a hero gains an ability that you do not understand what bonus it gives, then just right-click its icon to get a description. This comes in really handy when you first start out.