"Don't let anyone tell you what it is," because it's about people lying on Facebook.
Catfish is a 2010 documentary about a photographer, Yaniv Schulman, who starts receiving paintings of his work from a four year old girl. After becoming friends with her on facebook, he becomes friends with her family, specifically her sister with whom he develops feelings for. But then... their mother makes claims that seem suspicious, like her child is a painting prodigy that is being shown at an abandoned building. Instead of simply 'defriending,' they journeyed across the country. This confirmed for them that they were lied to, much to their amazement.
That's it. That's the entire movie. There's nothing more. No killing rampage, to everyone's disappointment (in more than one way). It's all built on the premise that people use facebook to lie. This is an entire movie that could be reduced to the following anecdote: "You know that girl who sent me a bunch of paintings? It turns out she was lying." That information was stretched to feature length and used to insult the intelligence of the $3,234,373 that fell for the secrecy of the advertising campaign. This presents us with two scenarios. Either the filmmakers are naive and exploitative or they take us for suckers. I mean, one look at the trailer could confirm this.
Warning: This article may contain spoilers.