I'm drawing much of this answer from my book/blog on the history of pornography and obscenity..

Indeed, I would agree with /u/Ungrammaticus that you probably need to be more specific in your question. Do you mean erotic books? Erotic poetry? or erotic film/movies? Let's begin with the word itself.

Pornography, Brit. /pɔːˈnɒɡrəfi/ , U.S. /pɔrˈnɑɡrəfi/ Hellenistic Greek πορνογράϕος (adjective) that writes about prostitutes ( < ancient Greek πορνο- (see porno- comb. form) + -γράϕος -graph comb. form) + -y suffix (compare -graphy comb. form), perhaps after French pornographie treatise on prostitution (1800), obscene painting (1842), description of obscene matters, obscene publication (1907 or earlier). (OED)

The usually-verbose Oxford English Dictionary simply says that it is a Greek word literally meaning 'writers about prostitutes.' It doesn't tell you that this word is only found once in Ancient Greek, where a certain Arthenaeus comments on an artist that painted portraits of courtesans. Then the word fell out of use for fifteen hundred years until it was used in 1842 to describe a treatise on regulating prostitutes and the erotic wall murals uncovered at Pompeii.

Walter Kendrick in The Secret Museum discusses how the uncovering of the ruins at Pompeii inspired the creation of ‘secret museums’ to house the discoveries. According to Kendrick, these museums (the first of which was the Borbonico museum in Naples) were only accessible to highly educated upper-class men, who could understand Latin and Greek and pay the admission price. As literacy rose and a marketplace of print developed in England and it began to seem possible that anything might be shown to anyone without control, then the ‘shadowy zone’ of pornography was ‘invented,’ regulating the “consumption of the obscene, so as to exclude the lower classes and women.”

Critics and moralists responded to the growing market, rising literacy, and the developing public sphere by expressing a deep anxiety over the impact and influences of erotic works. Erotic discourse began to be inextricably linked to a ’type’ of work that supposedly had undesirous effects upon the English public. In Lynn Hunt’s words then, “pornography as a regulatory category was invented in response to the perceived menace of the democraticization of culture.” As a category, then, it has a relatively short, modern history of about a century and a half. This is not to say that titillating works did not exist or were not understood as such—a claim that would be ahistorical, as the history of human perversity is as long as the history of our species. Instead, erotic works were not solely focused on sexual arousal at the expense of all else: erotic discourse was exactly that, a method of discourse (talking about things), and was usually linked with social, political, and religious criticism.

So, to summarize, then, the word 'pornography' itself is a really loaded term--it describes a sort of work that originated around the late eighteenth and early nineteenth centuries, a work that removes the cultural critique present in earlier works and just embraces the erotic. Before this time, there were still erotic works that might fit into out modern definition of 'pornography.' For example, the poetry of Pietro Aretino is quite scandalous:

He Lets’ fottere [fuck], my love, let’s fottere,

Since all of us were born only to fottere.

You adore the cazzo[cock] and I the potta [pussy].

The world would be nothing without this act.

If it were proper to fottere after death,

I’d say let’s fottere ourselves to death,

Then we could fottere Adam and Eve,

Who died such a dishonourable death.

She: Truly, if those truants hadn’t eaten

that treacherous apple in the garden,

Lovers would long ago have quenched their lust.

But let’s stop chatting. Stick your cazzo in

So that it reaches my heart, and crush the soul

That lives or dies issuing from the cazzo.

He: Don’t leave out my balls [don’t stop my orgasm] –

Take [it] inside the potta, those witnesses of every extreme pleasure

Or the Dialogues of Luisa Sigea:

Briefly: having placed his leg inside of mine, applying it first as a wedge, he thus made way between my thighs for his whole person. He mounted me, pressing his breast down upon mine; what could I refuse him then? this new and unusual weight inspired me with violent awe. He, while abating with one hand the fury of Priapus, applied the battering-ram to my gates

But none of these things would have been considered pornographic or pornography during their time, as they also spent a great deal of time and effort critiquing the Catolic Church, European morals, or upper-class noblemen. The first work to really seperate the social critique from just straight-up pornography is Fanny Hill

With such lovely descriptions as

Under my head, in ease of it; then spreading my thighs, and placing himself standing betwen them, made them rest upon his; applying then the point of his machine to the slit, into which he sought entrance, it was so small, he could scarce assure himself of its being rightly pointed. He looks, he feels, and satisfies himself: there driving on with fury, its prodigious stiffness, thus impacted, wedgelike, breaks the union of those parts, and gained him just the insertion of the tip of it, lip deep; which being sensible of, he improved his advantage, and following well his stroke, in a straight line, forcibly deepens his penetration; but put me to such intolerable pain, from the separation of the sides of that soft passage by a hard thick body, I could have screamed out; but, as I was unwilling to alarm the house, I held in my breath, and crammed my petticoat, which was; turned up over my face, into my mouth, and bit it through in the agony. At length, the tender texture of that tract giving way to such fierce tearing and rending, he pierced something further into me: and now, outrageous and no longer his own master, but borne headlong away by the fury and over-mettle of that member, now exerting itself with a kind of native rage, he breaks in, carries all before him, and one violent merciless lunge, sent it, imbrued, and reeking with virgin blood, up to the very hilt in me...

Fanny Hill was undoubtedly pornographic and obscene. So you might expect that John Cleland, it's author was immediately arrested and thrown in jail?

Well, you'd be wrong.

Cleland was first threatened with prosecution in November of 1749 and forced to pay fines, and then in March 8th of the following year, God himself prosecuted Cleland—at least, according to the Bishop of London. Bishop Thomas Sherlock, reacting to a series of earthquakes in London, wrote “A Letter on Occasion of the Earthquakes in 1750” addressed to the people of London. In it, he declared that it was his “heart’s desire and prayer to God… that you may be saved” from the “unnatural lewdness” England was immersed in, and he targeted Fanny Hill specifically as an “open insult on religion and good manners.” Cleland was again brought on trial, whereupon he disavowed the book and wished it would be forgotten—and even “with God in his side, the Bishop of London could not bring about a prosecution for a literary crime whose status as a crime was culturally undefined.”

This, failure would lead to the organization of religious groups, that would band together in the Society for the Suppression of Vice in 1802, and eventually get the Obscene Publications Act passed in 1857, right around the time 'pornography' was becoming a thing.