I havnt ever played a HOMM game but Im guess its a lot of the same thing. You pick a race you want to play (human, elf, orc, goblin, undead, etc.). You have one mighty wizard guy that is supposed to be you. You can also recruit "hero" units to join your regular armies that arent as bad ass as your wizard but are much better than your regulars. One scenario or level takes place on a large area map that has cities all over it, at least one of which is under your control. You can upgrade cities that you control to have different buildings and thus produce different units of your race. Build a wall around it so its easier to defend when attacked. You can also build a wizards tower that will extend the range of your spells so you dont even have to move out of it to nuke enemy units. You can even build a teleportation chamber in the tower so you can teleport to another one of your cities that has a tower. Of course you can always walk but thats for suckers.
http://en.wikipedia.org/wiki/Age_of_Wonders_2