EPS is Expanded PolyStyrene
XPS is eXtruded PolyStyrene
you don't need to understand the chemical formula, you need to know, what's important.
EPS = Expanded PolyStyrene (Styrofoam)
EPS is a rigid, tough and white, closed-cell foam made of pre-expanded beads. Its basic property for thermal insulation is its lambda value, which is depending on the density of the material.
For the purpose of ICF/PIF/StyroStones its density should be at least 25g/l, which gives a lambda value of 0.035, if properly moulded
You know EPS (styrofoam) probably well from trays for food or from cups to drink from, because it is not only insulating thermally well, it is shock absorbing, too.
XPS = eXtruded PolyStyrene
XPS consists of closed cells, offers improved surface roughness,higher stiffness and reduced thermal conductivity than EPS.
The water vapour diffusion resistance is better than EPS making it more suitable for a wet environment.
The price is higher
Neopor is a brand of BASF and it is EPS with the addition of graphite particles, thus enhancing the thermal insulation, which is about 0.031 (which is lower and therefore better than simple EPS)