(This node kind of sucks. Read Is pot addictive? instead.)
There is something of a difference.

Addiction is the more generic term; all it means is that you're doing something and you can't stop. The thing you're addicted to does not have to be chemically a narcotic. It does not have to be a chemical. It does not have to be harmful; the point is you can't stop. Whether you want to stop or not is unimportant, only that you can't. Marijuana, Heroin and a lot of prescription drugs are all addictive. So is chocolate. So is IRC. So is sex.

The difference between an addiction and a chemical dependency is most clear if you examine the difference between IRC and heroin. IRC, you do it a lot, and after awhile you just don't really want to stop. Awhile after that, you discover you do want to stop-- you're tired of it-- but somehow, every time you turn on that computer, your hand moves that cursor over that IRC client icon and double-clicks. Maybe you don't stop because you don't know how to stop; because you have nothing better to do; because you've come to get something out of it you can't get anywhere else. Either way, you have an addiction. You have a need. You do not have a chemical dependency.

Chemical dependency creates need as well, but to a much worse degree. And the difference lies in what happens when you violate that need.

Now think about what happens when you take heroin. You have actual chemicals (endorphins, or approximations thereof) going through your veins there, chemicals that aren't there normally in that intensity. And the chemicals make you feel good. IRC releases chemicals that make you feel good, too-- but those chemicals are supposed to be there. Those chemicals your brain put there, because that's how feelings work more or less. But the chemicals the heroin sets loose aren't supposed to be there, and they do funky things. Specifically they go and find your pain transmitters, and plug them up so the pain can't go through. So the pain stops. Physical pain, emotional pain, whatever-- it can't get through.

The thing this naively ignores is that your body is designed to route around damage. And if you have transmitters being plugged that your brain didn't plug, it will interpret that as damage and route around it. It will grow new transmitters.

You now have new pain transmitters that you didn't have before. Meaning you now transmit pain more efficiently. And so as soon as that heroin goes away and the transmitters are no longer being blocked, you will go into effectively the opposite of a heroin rush; you will now be more receptive to pain than you were before. This is what a low is. This is what withdrawal is.

Thing is, those new transmitters you just grew don't go away. They stay there. It's permanent. And that's what a chemical dependency is; you now no longer are able to function the way you were before normally. Something that before your body could supply naturally now has to be supplied by the heroin. And so you are dependent on it, not for something that the substance gives you, but for something the absence of the substance has taken away.

So you do IRC for awhile and stop it, and you have nothing missing; you are the same person; the only thing beckoning you back is the memory. Take heroin for awhile and stop it, and a part of you is gone; you chemically are missing an ability that you had before, because your body is giving out the same chemicals naturally but your biological need for those chemicals has been increased.

So the question i want answered: i know that pot and ecstasy are addictive. That much is obvious. But do they create a chemical dependency? If so, how much? If they do it to you at all, i would find it hard to believe they do it to a degree comparable to, say, the degree nicotine does it to you.
I've known people who have gotten into marijuana deeply and just walked away-- but i've never known anyone who has smoked cigarettes for more than a week and been able to stop. This is not exactly a definitive scientific test as to severity of dependency, but it does seem that the government has some odd priorities in terms of what addictive substances should be legal. I'm quite likely wrong, and if someone who knows what they're talking about would like to correct me i would be happy to listen..

StrawberryFrog: You make good points, but any disagreements you seem to be making with me seem to be based on different definitions of words and phrases. I was defining "chemical dependency" as a case where taking the drug leaves you biologically unable to function at the level you did before without the drug, something that heroin and nicotine both seem to do..

As for growing new pain receptors, i did not say this is what causes withdrawal or addiction or dependency or whatever you wish to call it-- i said that this was the case with heroin. I was trying to describe part of the process of a specific example, not the general process for all examples. I apologize for being unclear, and will rewrite this node when i get around to it. If the process i stated is not the case with heroin i apologize; however, it is what my biology teacher my freshman year quite clearly stated was part of why heroin creates such a need for itself after you have taken it. I tried to find some support backing me up on the internet, but failed in locating anything. I may be wrong, but if so it's because a biology teacher lied to me, so cut me some slack.. -_- sorry!