The basics: a turn-based strategy game in the vein of Civilization (develop towns, build armies, manage diplomatic relations, conquer enemies), set in a high fantasy world.
What makes it special, and worth your time even today, is the sheer amount of options and choices you can make. No other strategy game of this type gives you the freedom Master of Magic does. At the very beginning you can design your own civilization, selecting the attributes of your commanding wizard and the type of race you'd like to lead. You can hire heroes to lead your armies, and give them powerful enchanted artifacts to wield. You can learn new spells, which have effects as simple as doing damage to an enemy unit to opening a portal to a parallel "mirror world." The units you can command range from simple Halfling slingers to powerful demons, giant sea turtles that can carry armies on their backs, and flying griffins.
Having a version that will run smoothly on today's computers is a gift.