Just to expand a little on Gods explanation
Upvc has been around for roughly 30+ years and is basically a hollow section plastic extrusion, this is then cut into 5 metre lengths and shipped to double glazing fabricators who cut to size, weld corners together to form frames, a pvc door is made up of a frame into which a panel is fitted much like a double glazed unit fits into a window with glazing beads to all four sides, the panel is flimsy in comparison to a composite door as it can only be so thick so as it fits into the frame, most now are 28mm thick but years ago could be as little as 20mm
GRP and composite doors are pretty much the same as each other, as God pointed out these come like a solid door just like a timber door but they are made up of 3 elements - an inner pvc/grp face, a foam or solid timber core, and an outer pvc/grp face hence the term 'composite' and are between 40 and 50mm thick, ideally timber cored doors are better because they are stronger but because of this are more expensive than foam cored doors, thats not to say foam doors are rubbish, a little bit weaker yes but perfectly ok if you like the look of composite and are on a budget, composite are becoming very popular ATM and this will drive prices down due to competition
As for security IMO composite are the better door although pvc doors can be good too with the right glass or panel and top quality locking system. You've probably heard stories about lock bumping and lock snapping, these are very much real and not scare mongering so when choosing a door - pvc or composite choose or insist on better quality lock barrels, FYI you need to ask for anti bump, anti snap, anti drill and possibly even anti pick, again these will be a little more than a cheapy barrel the installer will supply but worth every penny, there is however a newish system on the market now made by lock manufacturer ERA called 'vectis', this basically replaces the weak point 'the barrel' and uses a back door style key like a chubb key as opposed to a yale style key that barrels use, very neat system.
Now all you need to do is choose a decent installer but that is a whole new thread
