Dungeons and Dragons is one of those games you can't cleanly explain to someone; they have to play it a couple of times to understand what it is. Cucumba hit quite a few nails in the head, as he should have, being a DM for as many years as he has been.
Most people are used to having "role-playing" experience as described in "role-playing games" restricted to dialogue trees and wandering through linear forests with pre-generated AI controlled enemies and being forced to adhere to a strict ruleset which allows you to either hit something, cast something, or use a skill as described in the game's manual. Real table-top role-playing, the original role-playing experience, is a much, much more complex and rich experience than this. Dungeons and dragons is that experience.
Say you come up to a pool of water in a dungeon somewhere. If you're me you walk the hell around it because you've had a DM that has scared you crapless by water in all forms. If you aren't, and you have to go through it to get across, you'll probably end up going in at some point. In a computer game your options are generally restricted to walking through anything they place on the ground. If it's a linear RPG that requires you to do something with the water than it wont even let you just walk into the water if you want to, it will stop you and make some kind of comment about how the water is special leaving you to attempt some action there (Final Fantasy style linear). In the computer game perhaps you walk in and 5 skeletons pop out of the ground for you to fight. You fight, you move on to the next puddle where you expect more skeletons. Table top RPGs are not like this.
In a table top RPG the best the DM will do without you asking anything is tell you that there is water in front of you in the cave. Maybe he'll tell you it looks or smells as stagnant as the air. Maybe he'll describe it in vivid detail to you. It's up to you what you want to do from there. If you're in a hurry you could just tell you "I wade through it quicklky and get to the other side" and perhaps you get part way in and something with razor sharp nails grabs your leg and pulls you down while a mace flies out of the water towards your forhead. If you're more cautious perhaps you take a closer look into the water and the DM tells you that you can see a glint of light off of something metal at the bottom. Perhaps on longer inspection he would tell you that you can see a bony fist clutching an iron dagger whose arm leads directly into the ground, or maybe you can see the glowing holes of two hollowed out eye sockets partially buried into the dirt.
Let's assume you didn't look closely enough and you didn't see the glowing eyes or the arm going directly into the ground. At this point you have options. Maybe you fire an arrow into the water and attempt to stir up anything that might be laying in wait for you underneath. Maybe you lean over carefully, take a mace, and gently swash around the water with it to see whether or something in there is waiting to grab you. Maybe after a couple of seconds of swashing around the water a big boney hand comes up and grabs your mace, pulling it into the water. If you fail a dexterity check maybe you can pulled in too and lose your mace. If you win maybe you pull back and break off a skeleton's arm, fist still firmly wrapped around the handle of your mace. Perhaps you struggle and begin to drowned, but your violent splashing alerts your team mates to come to your aid, but all they can see is limps flailing around violently. They have to try and pull you out because, unlike computer generated RPGs, the people trying to save you have to act as their role, which means they have no knowledge of what is attacking you, or how to hit it. It doesn't just light up with a red enemy circle when you look at it either.
And don't think those are your only options either. Maybe you want to try a running leap over the water, or you don't want to take any chances so you'd rather just send a lightning bolt into the water and fry anything that might be waiting for you. Or maybe you even want to put your head *into* the water to look for treasure or take the weapon from that skeletal hand, you masochistic, twisted ******* you. Or maybe, you've had a bad experience and expect a troll to lay in wait, so you pour the oil out of one of your lamps into the water and light it on fire with a flint. The skeleton jumps up and writhes in the fire for a while, then you smash it. You're feeling pretty good about yourself until the DM reminds you that you're in a cave and the oxygen is leaving almost as fast as the smoke is coming. Good job, ace.
In D&D you have real options, not just options that the game was written to understand, like walking and hitting things, or casting fireballs onto a random spot on the ground with a set area of affect and damage rate predetermined by the rule-set without fail. D&D is about the experience of putting yourself in the role and the setting of an individual of your creation in a magical world where you can ask questions and participate in a meaningful way. That scenario I just listed above might have taken you ten seconds in a computer RPG or a Final Fantasy game. Enemies spawn, you go into combat mode, and all that matters is that you kill them and collect experience. Well the experience in D&D is the event itself, and you could easiliy blow 10 or 15 minutes searching through the water or struggling with an enemy that caught you off gaurd in something as encumbering as water. When you gain a level, you should feel like you've actually had an experience and learned something more than about how many hit points a skeleton has.
In other words: DON'T CONFUSE D&D WITH FINAL FANTASY. DON'T CONFUSE D&D WITH BULDER'S GATE, and for God's sake, don't confuse it with everquest.
I just got told by my own wife :\
