You've probably watched a big-screen movie on television before and noticed a little disclaimer before the movie starts that says something similar to: "This film has been edited for television" or "this film has been edited from its original version." There's a simple reason for this and it's because the film, well, has been edited to make it suitable for television.
And there are several things that must be achieved before a feature film is ready to be shown on television. The first is to determine where to cut for commercials. Television shows generally have specific breaks and small cliffhangers written in to the scripts to make a cut to commercial more natural. It works well for TV, but a badly timed commercial break can really disrupt a movie's narrative flow.
Film editors also need to ensure the movie fits in the allotted time slot. Often that means cutting for time, but occasionally a scene that ended up on the cutting room floor gets slotted in if the TV time is longer than the actual film. Usually, though, the extra time is filled in with more advertising -- it's easier than finding deleted scenes.
Broadcasters also have to deal with aspect ratio. Now that we have HDTVs, aspect ratio isn't as difficult to deal with as in the past, but because movies are often shot with wide angle and long pan lenses, the ratios don't translate well to old-school TV screens. To compensate, editors often have to reduce the width of each shot to make it fit -- when it's done well it's difficult to notice in landscapes or close-ups, but occasionally it can even mean removing a character from a scene so that only their dialogue is heard.
The fun part comes when editors have to deal with obscenities. While it's not exactly illegal to show nudity or curse on broadcast television, networks are subject to FCC guidelines and can be fined if they fail to bleep swear words or show nudity.
Editors have two choices for dealing with profanity when it appears in a movie: They can simply mute or bleep the offensive language. Or they can take the more common option and actually dub over profanity with new dialogue. This often leads to some hilarious results as writers and editors struggle to find words that sound enough like the original to sync up with the actors' mouths but lack the offensive sting. Yippee-ki-yay, melon farmer!