I'm glad you asked this, because the list of differences is too long to type out. 99% of real sex is nothing at all like porn, so let's focus on the similarities.
1) Real sex often looks just as ridiculous as porn sex. This is why people never film themselves having sex, at least not more than once. Even if they're not having pretzel-sex (intricate positions), sex looks very unflattering.
2) Some people really are that loud when they have sex. Not everyone, but some people.
3) Some people, about 5% of the population, really do have porn star bodies.
4) Some girls (again, not many) actually will have sex with a total stranger. Those girls are usually only attracted to rich celebrity movie stars, pro athletes, and any kind of rock & roll musician.
And that's about it for the similarities, and they're pretty thin similarities at best. Everything else you see in porn isn't just a myth, it's a complete lie.
If a guy's wife / girlfriend catches him in bed with another girl, her immediate reaction will not be to join in. If a guy suddenly pulls out and blows his load in a girl's eyes, it will not bring her to orgasm. If a girl says "f**k you" to a guy and he says "okay, f**k me", she will not undress and get into bed with him.