Deadpool operates in the same universes as Xmen, but you rarely see any other Xmen in the movie (something Deadpool himself mentions). Sometimes there's a film or a book or a comic that breaks the fourth wall; Deadpool completely tears it down. Constantly. Other Marvel films are mentioned, and also constantly mocked; it's hilarious. It's a refreshing superhero movie, like Jessica Jones was refreshing, but in a completely new way. I've never laughed this much at a Marvel film, and the constant references to anything, from Reynolds' previous role to the lack of Wolverine.
The villain isn't like any other Marvel movies; he's not all powerful and Deadpool doesn't need the entire movie to deal with him. The film is made up of flashbacks, telling Deadpool's story when he was still only known as Wade Wilson, and his chase after the bad guy. Nice detail: the bad guys all have an English or a Russian accent.
"One thing that never survives this place is a sense of humour", as said in the trailer. Well, that's certainly not true; the entire film is filled with one-liners, references, jokes, hilarious moments, and so much more. It's hard to write a good review without giving anything away, which is why I'll stop here: You just have to see for yourselves.
And remember: stay for the after-credits.