Is styrofoam the same thing as expanded poly styrene pellets?

Depends on what you mean by same. Same chemical but completely different structure. Styrofoam as most of us thick of it is a bubble filled hard structure that requires complex molds and mixing to keep it from expanding too far. Bead board and similar products are basically expanded by steam and are much easier to handle - the mold can just be a box. But beadboard, which shows the bumps of the pellets is much weaker than the foam blocks.