"Obscenity" is that subset of pornography--sexually explicit speech--which is, according to Miller v. California,
The first two prongs of the test--prurience and patent offensiveness-- are determined with reference to contemporary community standards for the geographical area where the charges have been brought. The third prong, SLAP value, is determined according to a hypothetical "reasonable person" test.
Obscenity is not considered to be "speech" and therefore not entitled to First Amendment protection. Non-obscene pornography, however, is speech and therefore is protected, though if it is indecent speech in a broadcast medium it will still be subject to the form of regulation known as channeling.
The Miller standard has two significant problems. First, it is an illogical kluge which defines illegal material exclusively in terms of how people in local communities feel about it, rather than what it is. Secondly, by applying local standards to national or global media such as the Internet, Miller makes sure that the standards of the most conservative community rule the world.
In recent years, virtually all obscenity cases have involved visual images--photographs or films. Not too many decades ago, New York prosecutors hounded comedian Lenny Bruce to death and publishers of Henry Miller, Vladimir Nabokov and James Joyce, among many other authors, were prosecuted for their "obscene" literary works.