Black Christmas definitely deserves its reputation as one of the best horror of films of the 1970s, as well as being an early and very influential example of a slasher movie. It's more subtle and understated than a lot of its followers, though.
The plot involves a bunch of sorority girls getting threatening and sexual phone calls, which are effectively weird and disturbing.
It's hard to explain without spoilers but eventually there's a reveal that serves as an example of an extremely famous and common horror film trope/cliche. This is surely one of, if not the earliest, uses of this now incredibly cliche plot point, and I always find it interesting to see one the first uses of any well-known film trope. I'm sure anyone who has seen the film would know what I'm referring to.
This is also one of the first big examples of a horror film titled and based around a specific holiday or day, like Halloween, Friday the 13th, Prom Night, My Bloody Valentine, and countless others.
The villain in Black Christmas is great and one of the creepiest slasher bad guys in film history. There's even an abortion subplot, which is pretty daring for it' time.
The ending is fantastic too and features a very eerie final shot. Overall, the cast is pretty good, and there are a few familiar faces you might recognize, like Margot Kidder, who played Lois Lane in the Superman series. There's also Keir Dullea, who was the main character in 2001: A Space Odyssey, and John Saxon from Enter the Dragon, Dario Argento's Tenebrae, and A Nightmare on Elm Street.
I highly recommend watching this to anyone who has even a passing interest horror, and it's a must watch as soon as possible if you're a slasher or 70s horror fan.
Black Christmas was remade in 2006, but I haven't seen that version yet.