I used to smoke. - Here "used to" means that you no longer do something that you once did on a regular basis in the past. When "used to" is used in this way, it is followed by the base form of a verb. Here, "smoke" is a base form. It is what you "would do" in the past, but no longer do now.
Here are a couple more examples:
I used to eat at at McDonald's. I don't anymore.
I used to speak only one language. Now, I speak English and my first language, which is Spanish.
I am used to smoking. - Of course, this means that you are accustomed to smoking. When "used to" means that something is not new and you are accustomed to it, it can be followed by a gerund or a noun phrase. In this case "smoking" is a gerund.
Here is an example with a noun phrase.
I am used to the smoke in crowded nightclubs.
"the smoke in crowded night clubs" - noun phrase
And here, once again, "used to" means that you are accustomed to something, and it is no longer strange or new to you.
"Used to" can also be followed by a noun that is not a verbal (gerund=verbal).
e.g. I am used to airplanes. - airplanes - noun
I am used to flying in airplanes. - flying in airplanes - noun phrase
I am used to flying - flying - gerund/noun = verbal