... between movies that are intentionally made bad and movies that strive to achieve greatness but are bad nevertheless.
Anyone walking into a John Waters movie believing that they are about to witness cinematic artistry is either deluded, ignorant, or misinformed. "Pink Flamingoes", for example, was quite deliberately made as a gross-out film. The same goes for most anything made by Andy Warhol.
"Plan 9 From Outer Space", on the other hand, was earnestly (if ineptly) made. It's terribly bad, but funny - although that humor is balanced if you know that Bela Lugosi, who starred in the film, was desperate for any income at all to feed his heroin habit at the time (and died halfway through filming).
Then there are movies that are made badly with tongue in cheek - "Evil Dead" and others.
Finally, there are those films that are made seriously, but are just bad. "Battlefeild Earth" has been mentioned many times. While I didn't pay money for it, I did stop John Carpenter's "Ghosts of Mars" at the point where Natasha Hestringe, having barely escaped an attack by Martian zombies, convinces her fellow survivors (against all logic) to go back and face the zombies again. I immediately trashed that movie. Similar, "Scary Movie II" I found deliberately offensive, as if the movie was crawling with filth.
Some of the objections in the posts below are founded not on a movie being bad so much as it being long or slow-paced, which I find sad. A movie in which things do not threaten to explode every ten minutes is not a bad or boring movie if it succeeds in making you think.
Personally, I've never walked out of a movie I've paid money for. I always remain in my seat, hoping (against hope) that there will be some new plot development, some interesting camera angle or line of dialogue. The nearest I ever got to actually leaving the theatre was during a screening of "My Own Private Idaho", which I found pretentious, empty and dull - but I waited it out anyway.
I think there are a lot of bad movies inflicted on the teen market simply because it's thought they don't know any better. I remember seeing "Band of the Hand" (with a theme song by Bob Dylan, no less!) during its initial theatrical run and barely staying in my seat - I remained mostly because it was the first film of a triple feature.
Nowadays I solely go to the cinema to see movies for the theatrical experience which I can't duplicate at home. A "Star Wars" or "Lawrence of Arabia" is designed to be seen 40 feet high, with THX sound. But an Eddie Murphy flick I can watch on my television (or more likely, computer monitor) without losing much. It also means I tend to lose far less - both financially and timewise - if I decide the movie is not for me.
-- "Don't criticise. Create a better alternative."