I always imagined that the difference is intended purpose.
Pornography is intended to increase sexual desire and increase excitement within the reader. Erotica is usually more literary and has other goals ALONG WITH producing sexual excitement in the reader.
I mean, Brazzers is for people to masturbate to. Lady Chaterley's Lover is highly sexual but is really a commentary on early 20th Century views of marriage and ideals of intimacy, plus much more.