Short version: It's Doom, perfected.
Long version: Heretic is, essentially, a professionally made Doom mod with a fantasy theme. In many ways, its status as a mod is obvious: the crossbow functions exactly the same as the shotgun, the Phoenix Rod is a rocket launcher, and so on. It plays like Doom as well: the game is broken into multiple episodes of 8 levels + 1 bonus. You go through collecting keys and finding secrets until you reach the exit. Killing anything and everything you can along the way, of course. Many of the items are similar as well: maps, invisibility, invulnerability, etc. etc.
This isn't to say that Heretic is just a re-skinned Doom wad. Heretic introduces brand new enemies and items. While many of the enemies are tweaked Doom enemies, there are new ones, such as the Iron Lich and the Ophidian. New items introduce new concepts, such as flight and emergency teleportation. Plus you can turn your enemies into chickens, and who doesn't love doing that? There is also the Tome of Power, which alters weapons for a short time. Some simply become more powerful, but others have different functionality. For example, the Phoenix Rod goes from a long range rocket launcher to a short range flamethrower, while the Gauntlets let you suck the life out of enemies.
Where Heretic really shines isn't the gameplay mechanics, but the level design. First of all, they are just gorgeous. Second of all, they are a technical accomplishment as well. The use of shadows and texture is excellent, which really creates a completely immersive atmosphere. You have everything from buildings, to narrow caves and alleys, to massive fields. The enemies are placed to work best with the current situation, so you always have a fight to deal with before you can admire the view.
Overall, Heretic is a bit harder than Doom, though the difference between the very easy and very hard settings is enormous. On very easy, as long as you don't forget to pause the game before you wander off for lunch, you probably won't die, though there are a few spots where you can take quite a beating. On the highest difficulty, it makes Legendary mode on Halo look like a walk in the park.
There are only one issue: the game was made at a time when 800x600 was considered a high resolution. If you try and play it through Steam, you'll barely be able to tell what's going on because it's so pixelated. After buying the game, download ZDoom, which bumps up the resolution and offers a few other features. It's a free download and setting it up consists of copying the Heretic IWad file into the ZDoom directory.