As a disclaimer, I am not terribly familiar with the Green Lantern comic books.
The Green Lantern had awesome special effects, in particular the bit where aliens flew around and punched things in space. There were also several things worth complaining about in The Green Lantern, notably the fact that there was only one female character with any lines at all (and she was a boring love interest on top of it), and the length of time spent on boring, cliched interpersonal interaction instead of aliens flying around and punching things in space. However, on top of that, the core long-term plot of the Green Lantern universe, which they introduced in the film and blatantly intend to use for sequels, was among the most nonsensical things I've ever seen in a film of this genera.
Let me sum up.
So once upon a time there were these immortal aliens called Guardians who decided that the universe needs policing. So they somehow harness the Power of Will, which is green for some reason, and give it to a bunch of people via these lanterns and rings. Bam, Green Lantern Corps created.
This seems to go well for a while, but then they are like "What if we started using a second Power along with Will?" It is unclear why they need a second power, but my guess is that the Power of Will seems to be the Power of being a Rash Douchebag.
Anyhow, the power they are debating is the Power of Fear, which is yellow. All of the Guardians decide it is bad news, except for one guardian who decides to test it out anyway (prob because he is a rash douchebag). Anyhow, it turns out that the Power of Fear is actually the Power of Creating Super Villains, and the guardian turns into this big yellow cloud named Parallax.
Don't worry though, they throw him in prison in the Lost Sector. Nothing could possibly go wrong.
Fast forward to the present in the film, when something goes wrong and Parallax has escaped his prison and is eating planets.
This one Green Lantern, (I think his name is Sinestro; nothing can go wrong if you have a name like that) who seems to be the leader (and in any case is a the rashest of douchebags) goes to the Guardians and is like "What's up with this giant yellow cloud; I have never heard of it" (again, why he has not makes no sense) The guardians, however, tell him nothing (which seems like a bad plan), so he goes flying out to fight the yellow cloud with a bunch of Green Lanterns. Unfortunately, being a rash douchbag does not work out for him, and various Green Lanterns die.
Sinestro then goes back to the Guardians, who tell him the above tale of Parallax. He then immediately concludes that the only way to defeat the Yellow Power of Creating Super Villains is to create a yellow ring to harness it. For some reason the Guardians are ok with this, and do so.
However, the new Green Lantern from earth, who has been of having boring cliched conversations with the only female character in the film, flies in at the last minute and is like "You can't use this yellow ring you just made that I know about for some reason; instead, come help me stop this yellow cloud from eating earth".
In response the Green Lanterns are like "Nope, we're giant douchbags; fight the yellow could yourself".
Fortunately, the law of inverse ninjas applies, and the giant yellow cloud that killed a half dozen Green Lanterns is defeated singlehandedly by the least experienced of them.
He then comes back to Green Lantern Land, and they have a giant Green Lantern Party and he is a hero.
Cut to credits.
After the credits, Sinestro is standing around with this facial expression like "Whelp, better make sure there is a Villain for the next movie" and puts on the yellow ring,
Fin.