"9/11 was an inside job" is a phrase that rose in popularity on forums and websites in the years following 9/11. Most of the talking points come from a small hand full of video "documentaries" such as "Loose Change" that circulate around the internet. Along with Moon Hoax conspiracies, super secret dominating new world order conspiracies, and JFK conspiracies, it supports itself on a set of lies, distortions, myths, and debunked crap.
People who believe it was an inside job call themselves "truthers." Yeah, that's right, "truthers." The worst thing you can do is get sucked into an argument with one of these "truthers." The are impervious to logic and devoid of intellect. And even if you manage to cure one of their misbelief, five more will spring up the next day. While at first glance it might seem like these "truthers" are mostly liberals or democrats, it seems they include a large number of conservatives or republicans as well. The liberal population is mostly kids who will grow out of it at some point, but the right wing population is usually middle aged loonies suffering from some kind of extreme paranoia illness.
Observe the post by Krystem above. This kid brought up 5 regurgitated talking points that have been repeated ad nausea over the years. And the answers have always been patiently explained to them. They haven't gotten it yet, and they probably won't get it ever.
The bulk of conspiracy arguments work like this: "X is impossible for me to understand therefore there is a conspiracy." Perhaps the favorite claim by conspiracy nuts is: "Jet fuel burns at a temperature not hot enough to melt steal therefore there is a conspiracy" Don't bother trying to tell them the difference between temperature and heat, or that the fires burned many things besides jet fuel. In the end, the buildings collapsed not because of melting steal anyway. The buildings collapsed because the fire weakened, not melted, the steal. Under extreme heat, beams warp, bend, buckle under load, and joints and bolts become stressed. It doesn't have to be hot enough to melt the steal. It only has to be hot enough to weaken the structure enough that it can't support itself.
9/11 inside job? Forums on the 'net are crawling with twats who think so