Okay, I admit, I've done this skill many times before I cam here to write about it, but it's just so much gosh dang fun! It's basically a skill that can involve every other skill! You can mine then make armor, you can woodcut and do firemaking, runecrafting, every single skill! Even members skills, but I'm not a member yet. :(
But, the whole skill is about going through dungeons, skilling your was through them, killing the boss, and doing it again. As you level your Dungeoneering, you can go deeper and deeper into the dungeon, fighting harder and harder monsters, while needing to have higher and higher skills. My Dungeoneering is only 16 right now, so I can access dungeon level 7. There is also a Complexity rating, where there are 6 levels, and the higher it is, the more complex it is. Simple as that. Oh, and if you use really low complexity levels, you get big XP deductions at the end. Only at complexity level 6 will you not get deductions.
Here are some screenshots of Dungeoneering: