Okey.. here it goes.
Plexiglas = Cheap and easy. BUT! be careful when you cut it. Use a saw and with a steady support. If it is allowed to flex to much it WILL crack. You can use a rotarry tool to cut it but chances as you'll just melt it the plastic and make a mess. Plexiglass can be bent by heating it up but again, be careful, it bubbles really easyly when heated to much and too intensly.
Fibreglass = Comes in sheet of fibresmats or incased with epoxy. leave this alone. Unless you are gonna reinforce something or mold something, leave it alone.
PVC = A hard plastic. Kinda like an opage plexiglass but more flexible. It will "whiten" when you stress it too much. (unlike plexiglass which just cracks)
Sintra = A.k.a Foamed PVC (and in certain places FOAMEX). Just like PVC but the it has been processed so there is pockets of air inside the plastic. (on a microscopic level) So the plastic cells are larger than in regular PVC but still has the same properties. Bends easeir than PVC and does not crack. It does however produce a fine dust when you are working with it so use googles and mask. (as should you always) Cheaper, lighter and more worker friendly than pvc. Does however produce poisinous gas when heated so stay in a ventilated room as well.
Good luck!