The Guardian has a nice short summary article that debunks the 9/11 truther claims, here is a quick extract …
The twin towers were destroyed by controlled explosion
Truthers say video footage of the buildings falling points to demolition due to the way the towers bend before collapsing. Also, there appear to be explosions as the windows blow out, floor by floor, from the top downwards. One US academic claims to have tested samples from the wrecked towers which show the presence of chemical residue, suggesting explosives had been used.