Gang bang porn is the kind of porn that you can watch for free. It's also the kind of porn that involves an entire group of men having sex with the same woman. There is usually a girl surrounded by at least 10 guys who take turns fucking her. Running a train is also another term for gang bang.
A gang bang scene will usually start with the girl sucking all the guy's dicks and getting fucked by them one by one. After they're all hard and horny, the next thing you'll see is the girl being double penetrated. Sometimes they get spit-roasted. Double stuffed. There are just so many positions where she is fucked by more than one cock at a time.
When they get tired, the girl will give them blowjobs again. When she's ready, they'll start running a train on her again. There are many different gangbang porn subgenres, including, for example, interracial gangbang. That is when a white girl gets fucked by black men. It's very rare to find a white guy getting fucked by a group of black women, but there are reverse gangbang porn movies as well.
Sometimes they run a train on the girl until she is sore. She'll have to take a break and come back again. Sometimes, it's all about a blowbang, i.e. a gangbang with just blowjobs, and no vaginal penetration. Sometimes they make the girl squirt. There are just so many things you can do with a group of men and a girl.