If you want some fun, go on YouTube and search for "Canadian Interview with Stephen King", in which he plugs this movie. It will show you the state he was in at the time, and goes a long way toward explaining why this movie is what it is.
Presumably, it all began when Stephen King went to the studio and said "I'm Stephen King, and I'd like to turn this 3-page short story into a full length movie, and I also think it would be kind of fun to direct it". The studio head said "sure", and they celebrated over a few lines of cocaine.
Then King went off and did more coke, then more coke, then way more coke, and then made the movie while taking frequent coke breaks - which was OK, since by then he would have had no need for sleep. Seriously, this rivals even the legendary Star Wars Holiday Special as a cautionary tale about the dangers of cocaine in the entertainment industry.
The premise of the movie is that machines suddenly become sentient - and mean. The title card indicates this is because the earth is passing through a comet's tail. Then, halfway through the movie, one of the characters floats a completely different theory. Things are tied up by a very confusing end card that seems to indicate that somehow both theories are true.
The good news is that none of this matters in the least. Basically, it's about machines trying to kill people, but being pretty bad at it. When I say "machines", I mean mostly semi-trucks. There are a few other killer machines, mostly thrown in for comic relief, like a soda machine that shoots out cans hard enough to kill people, but it pretty quickly settles down to trucks - one of which inexplicably has an evil clown/demon face on it. These trucks gang up on a way-too-large cast of characters holed up in a truck stop, and proceed to .... mostly just drive around in circles.
In fact, when you think about it, trucks are a pretty terrible killing weapon, because they're really easy to dodge. Cars would be much more effective, but I guess he didn't want to be *too* obvious about recycling Christine. Luckily for the killer trucks, anyone being chased by a semi truck - you guessed it - runs in a straight line.
That's pretty much it. Nothing really makes any sense. Some machines are evil, but the protagonists frequently use machines against the other machines, and they never suddenly rebel. Sometimes the machines can violate the laws of physics. For example, an electric knife can levitate to cut someone. On the other hand, a gas pump is unable to levitate the nozzle to pump gas. They need people for that.
Anyway, lots of people die, usually with Monty Python-esque amounts of blood. Apparently, King had put in some *really* gruesome scenes that had to be cut to keep from getting an "X" rating. Lots of things explode. There's one very softcore romance. That's about it.
In the end, it's fun to drink a few beers, crank up the AC/DC soundtrack (probably not as loud as it was playing in Stephen King's head) and catalog all the ridiculous plot holes.
Also, have fun trying to make a young person believe that they once showed movies like this in theaters.