Dark Souls plays more like an Action game/ 3rd person 3d platformer than it does a conventional RPG. It has loads of RPG elements but lacks narrative aside from item descriptions and npc dialogue (both of which fairly easy to miss), theres not really any 'towns' other than a main one used as a sort of hub area. It does have stats, spells, equipment, and so on (technically party members as well) which kind of make it an rpg.
Gameplay in Dark Souls is largely dodge-based with a stamina resource restricting how often you can dodge, attack, and other combat actions. Weapon types have different movesets (somewhat similar to fighting games in a way), as do damaging spells have different styles.
When you die, you leave behind however many souls (which is basically both money and exp. in the same currency) you had when you died. Then you respawn at the last bonfire (the game's checkpoints) you visited and to get the souls you lost back you have to pick them up without dying. Basically, what makes the game considered difficult is that its fairly easy for the enemies to kill you, so you have to replay an area until you can reach the next place without dying. However, its not a Roguelike game so you simply pick up where you left off rather than start from the beginning again.
The real hard part, IMO, is completing npc storylines. Without spoiling too much, basically you need to get npc party members through certain areas and boss fights without getting them killed. Because when npc's die in Dark Souls they dont come back. but its all optional whether you want to bother with them.
There's also multiplayer, you can help out others with fighting monsters and bosses, or invade other games and try to kill people, or even just 1v1 duels, etc.