I wrote a big response to this which the hamsters ate last week. Here's a short version:
"Obscenity" is the only of these terms with real meaning, at least from a constitutional standpoint. In a legal sense, obscenity is material so offensive that it is outside of the protection of the 1st Amendment. Exactly what qualifies as obscene is based in part on local community standards, so what qualifies in Utah might not in Times Square. State and local governments have the power to prohibit obscene matter and make it illegal to possess it. Note that obscene material isn't automatically illegal -- it is contraband only if the local government has so acted. The point of obscenity is that local governments have the power to prohibit it if they so choose. Governments cannot prohibit the creation, sale, or display of non-obscene matter, even if they wanted to. However, if matter is obscene, governments can ban it almost completely, possibly making it illegal to possess or purchase, even in private amongst consenting adults.
Note, however, that non-obscene matter which is offensive, unsettling, pornographic, ostensibly harmful to minors, or so on, while still protected under the 1st Amendment, is subject to reasonable time, place and manner restrictions, giving the government power to limit it such that those who seek it out can get it but those who don't want to deal with it can shield themselves and their children. In my limited understanding, the FCC's concept of "indecency" is a time, place, manner type of restriction -- while the government can't prohibit "indecent" material which doesn't rise to the level of obscenity, it can limit its presentation on the public airwaves, which are granted to broadcasters in order to facilitate the public good. Indecency clearly isn't the same as obscenity -- according to the FCC, a split-second view of a single nipple framed in a long shot is indecent, but it's clearly not obscene (and the Supreme Court agrees, having ruled that Candace Bergen's boobs are protected by the 1st Amendment).
Pornography generally isn't obscene, although since community standards enter into the definition of obscenity, some porn probably qualifies in some places. As the Internet makes a wide variety of pornographic matter available to everyone everywhere such that even the most extreme depravity becomes blase, this may become an area of the law which is revised.