You've got a hero who has an army. You fight enemies (usually random wandering monsters) with your army. You get experience to level up which give you more runes (basically skill points tied to one of three specific tech trees) and lets you choose a bonus to one of your primary statistics. There are a ton of stat-boosting items around the map, probably most significantly the ones that boost leadership which let you field a bigger army. The general idea seems pretty awesome, especially to anyone who has played Heroes of Might and Magic and wished some of the "no enemy kingdom" missions were a bit more involved. If your troops are killed you must go recruit more somewhere else... and your troops *will* be killed, as you fight quite a few enemy armies that contain large numbers of lightning bruisers and/or archers. You can complete fairly standard "deliver x item to y location" quests and even more "kill that big pack of hyenas/gang of pirates/death knight" type quests. One important thing to note about the game is that the resources available to you are finite - unlike Heroes of Might and Magic there is no income, contrary to what the difficulty selection screen suggests.
Unfortunately the game breaks down pretty badly on higher levels of difficulty. In battles, if an enemy stack gets to your army at full power you will take tremendous casualties no matter what kind of defense you have. So, instead of fielding well-balanced armies including a flier, an archer, a spellcaster, and a couple of tough melee units, the best army is generally composed of five archer type units. Instead of exploring the areas you are in completely, it becomes necessary in the first quarter of the game to skip ahead, dodge all the monsters in your path, and steal all the powerups lying around before the next mandatory boss battle (which you have absolutely no chance of winning at that point). This will probably take five hours, which is entirely too long to spend scurrying around picking up flags. After you've done this, you can go back to areas you visited hours ago and then start fighting. It is still definitely beatable, but the strange gyration you must perform to complete the game will make want to quit long before this marathon of a game is over. Even on lower difficulties, the best strategies make almost no intuitive sense and ignore many half-baked gameplay elements.