Dark Souls is a 3d action-rpg in an open world with monsters, big and small, that need to be defeated. The best way I can explain it is Legend of Zelda meets Metroid with an infusion of Ghosts'n'Goblins/Contra/Ninja Gaiden/<insert the hardest game you've ever played>. There's a giant world to explore, giant bosses to slay, and treasure to be found all over the place. However, unlike Zelda and Metroid, very few zones are actually locked for you at the beginning. Yes, there are locked doors between shortcuts that you'll unlock later, but for the most part, you can wander down any path, and the only thing that will stop you is a giant axe to the face from an NPC that says you shouldn't be here. It's not like you need the ability to turn into a ball or have the hookshot to get there.
Now, something that's going to happen to the player is death. And it will happen a lot. Luckily, death has very little consequence. You'll drop all your humanity (necessary for returning to human form) and souls (currency and experience), and be returned to life at the bonfire you last rested at as an undead with all your equipment intact. From there, you can go ahead and recover your dropped souls. But be warned, all creatures have been reset.
Combat is definitely the star of the show in Dark Souls. There's bows, magic, swords, axes, whips, spears, and more to find, use, and upgrade. Each has a unique style of combat, so there's plenty of room for you to find what works best for you. Would you like to hold your shield while prodding the enemy with a spear? You can do that. Do you want to go all out and two-hand a powerful greatsword? Go for it. Each style feels unique and viable. Weapon and armor upgrades are accomplished through the various smiths found in-game.
Combat allows you to roll around, block, dodge, and parry in an effort to gain the advantage over your adversary, which you'll need to win. Even the lowliest of enemies can decimate a reckless character. Strategic combat and careful exploration are encouraged when first entering an area to allow the player to make the most progress towards the next boss or bonfire.
Boss fights usually require a few deaths before you understand the timing required to block, dodge, and attack. If, however, you find yourself stuck on a boss, you may need to level up, boost your equipment, or summon a friendly player or NPC to help you. Watch for the glowing white signs on the ground while you're in human form. You'll be able to summon a player character's phantom to help you fight.
Of course, while in human form, you'll also be subject to invading phantoms. In a few cases, the phantoms are NPCs who will drop decent loot upon death. However, invading phantoms will most likely be other players. They're out to slaughter you for laughs, humanity, and souls. Unfortunately, the only time it's particularly bad is early in the game when you are most likely to need the most help. Low level players who fought to the end-game are too happy to invade the first few levels to kill new players.
The final multiplayer component is the ability to leave messages on the ground. Is there a trap you'd like to warn people about? You can do that. Signs telling you to be careful, where the next bonfire is, and any illusory walls are all over the place. At the same time, however, you'll have to be wary of the people who put false messages to trick or kill any unsuspecting characters. For example, I jumped in a well in the 2nd area of the game because someone said treasure. I only found death. Thankfully, you're allowed to rate messages as + or - to let others know if the message is true or not.
Now, something that many find frustrating is the almost complete lack of direction from the game. You land in Firelink Shrine, and you are told there are two bells to ring. One above and one below. After that, you're almost on your own with a few cutscenes to help you out after major events. If all else fails, try talking to NPCs, checking vendors for important items, read the description for any keys you may have, and go exploring. After ringing the bells, you should have enough shortcuts and familiarity with areas to be able to cover ground quickly and look for new paths. There's also a fantastic wiki out there.
All in all, Dark Souls is an incredibly satisfying game. The difficulty, though, makes this title a hit or miss for people. If you're unsure, I'd recommend waiting for a sell on the various digital distribution sites and picking it up for $10 if you're curious at all.