I think that the primary difference between a film and a television show is like the difference between a short story and a novel. A film is like a short story, one that you could enjoy in one sitting, while a television show, like a novel, gets enjoyed over a period of time–each chapter like a new episode. A TV show is more of a commitment with episodes that bleed into one another and characters and story lines that you follow for the remainder of the series. Television allows writers to fully develop characters and allows audiences to see the complete trajectory of a character. Films undoubtedly develop characters and storylines but not with the same complexity and temporal space as television shows. A movie has two hours to develop, peak, and resolve while a TV show has almost an unlimited amount of time to do so.
There’s also a difference in the way each medium is viewed. When watching a movie, you sit in a theater in front of a big screen, surrounded by dozens of other viewers (unless you rent the movie, of course). When watching a TV show, you are in your own home, perhaps in your pajamas, leaning back on your couch. While films provide the experience of being out, spending money, and passively socializing with others, television provides a more personal experience, one that you can actively enjoy with your close friends or family (or alone) in a more comfortable and private setting. You can go to the bathroom, make a snack, or perhaps talk with your friends about what you are watching during the commercial breaks. Or you can just sit contentedly by yourself, watching your favorite show one episode after the other.
Though there are differences between television and film, the two mediums are becoming more similar in quality than ever before. In the past decade or so, the quality of television is becoming more comparable to the quality of film. With shows like The Sopranos, Mad Men, Breaking Bad, Six Feet Under, etc., television shows have become more complex, more psychological, and better-written overall. A lot of contemporary TV shows are even better than a lot of contemporary films. Which would you rather watch: Mad Men or Cameron Diaz’s The Box?