It's a difficult line, I agree with hkkmr in the obscenity versus art.

But to expand further in my own way
I think the goal or intent of pornography is its explicit and is there solely for sexual gratification of the user
I think the goal of erotic art however is to express sexuality which can have an impression and is not always explicit

This of course has overlapping because erotic art may intend to express something about human sexuality but function as an aid for sexual gratification, while pornography may intend to be used as an explicit aid for sexual gratification but actually invoke an impression in the viewer and actually function as a tool towards the expression of human sexuality.

The real line is drawn as a subjective agreement of the individual and society, certain types of sexual media are considered by the vast majority of society to be obscene and there solely for sexual gratification, while the vast majority of society sees other sexual media as less focused on sexual gratification and more focused on the expression of human sexuality. While certain stuff like a picture of a nude woman sketched is usually considered art on one extreme spectrum and hardcore throat fucking porn dvds considered pornography at the other, inevitably there is always going to be a hazy boundary that has to be crossed where society is fractured. Further there are always going to be individuals as outliers that see the most extreme pornography as art and even the most classy rendition of a nude woman sketched as smutty pornography.

The division itself is a mute point as it is what it is, but the label matters only in the social context of the situation. Pornography is usually regarded as a tool used for a person's entertainment or sexual gratification, its explicit, and obscene, but usually not smutty. Erotic art is usually regarded as something which expresses sexuality and make evoke feelings in the viewer but is not intended for sexual gratification or at least in an explicit sense. Smut is usually regarded as a more gritty form of pornography in which the sexual gratification is derived from something especially taboo, disgusting, forbidden. Not all "smut" may actually involve abuse or malicious intent, as it may be a consensual act in which actors depict something taboo. It can even be argued that smut is in fact a sort of art in order to challenge taboos. Ultimately its a difficult determination as an absolute law with fixed rules.

Not to further mess with your minds but consent is a shady issue as someone who is especially traumatized or abused may feign consent so as not to upset their abuser, something like stockholm syndrome. Society then may claim they need to determine the correct course of action in place of the individual, but then one could always argue that society itself is just another abuser trying to brainwash the individual towards their own end.

This is why I say ultimately its a subjective relationship between the individual and society. Not all societys or factions or subfactions or individuals may regard this all the same way, and an individual themselves can be conflicted as to where that hardline is drawn.

Usually for me I regard something explicit to the point of leaving little to ponderance or the imagination pornography, I regard anything which makes me feel sick as smut, anything which makes me seriously question the welfare of the participants as malicious smut, and anything which leaves a little to the imagination and doesn't explicitly show sex acts as erotic art.

Generally I deal mostly with what I consider to be at the boundary of erotic art and pornography, I usually avoid things I feel smutty as I don't think its good for me to travel into that realm, and I will sometimes push more into the erotic art side when I feel like I need to use my imagination more and feel desensitized by overly explicit things. Also I tend to be more liberal about sexuality than the typical American Conservative or Christian, but don't feel like my interests in sexuality push the line compared to the typical American Liberal. I generally view erotic art as useful in the process of understanding human sexuality at a deeper level and in helping stoke creativity and libidinal energies, and generally regard any participants with respect. I generally view pornography as a tool to relieve excess sexual tension and don't value it more than actual relationships with people or things which I consider art. When I encounter smut, I usually head in the opposite direction but view the experience as a darker exploration of taboos -- I also look at filming executions as another form of smut, this acting on the death side of the id, rather than the life side of the id.

That's just me, like I said its mindfuckingly complex and usually I don't like to think about it too much because my head will start to spin out of confusion.