The Apartment is a comedy. But like most classic comedies, you probably aren't going to laugh too often. It's a comedy because of the writing (which is smart as hell) and the acting. Billy Wilder could never be over-praised. This film is the only the second of many films in which Wilder used Jack Lemmon.
Lemmon stars here as Mr.Baxter, an employee for an insurance company. He's an asset to the company as a good worker, but more importantly as a tenant in a fine apartment. For, you see, many of the higher-ups in the company have...relations...with women who don't happen to be their wives. Although a hotel might work occasionally, Baxter's flat is quite convenient for everybody. In return, Baxter gets praise from the higher-ups. Although his apartment might be occupied at inconvenient times for him, the return is worth it, and since he has no love life at the present, he's not losing much valuable time. Of course, this is until things get complicated. Shirley MacLaine and Fred MacMurray star alongside Lemmon, MacLaine being Lemmon's love interest. Saying more might ruin a few plot points, so I'll change the subject.
It did very well at the Academy Awards, and took home Best Picture as well as Best Director, Best Film Editing, Best Writing directly for screen and Best Art Direction and was nominated for Best Actor (Lemmon), Best Actress (MacLaine), Best Supporting Actor (Jack Kruschen), Best Cinematography, Black-and-White and Best Sound.
I highly recommend this movie. You'll enjoy it much more than any new $6 dollar action rental at Blockbuster. The writing is timeless. This film was the last black and white film to win Best Picture until Schindler's List won in 93, you can tell that color is on its way. The background of the story is a group of businessmen casually cheating on their wives. The Hays Code days are over. The story is much more entertaining than 99% of the stuff out there, and that's why it's a classic. And after all, you can never see too many classics.
IMDb.com was used for the details. Just like every other movie writeup on this place.