The Glass BSDF is used to add a Glass-like shader mixing refraction and reflection at grazing angles. Like the transparent shader, only pure white will make it transparent. The glass shader tends to cause noise due to caustics. Since the Cycles path tracing integrator is not very good at rendering caustics, it helps to combine this with a transparent shader for shadows; for more details see here.
Color of the surface, or physically speaking, the probability that light is transmitted for each wavelength.
Influences sharpness of the refraction; perfectly sharp at 0.0 and smoother with higher values.
Index of refraction (IOR) defining how much the ray changes direction. At 1. 0 rays pass straight through like transparent; higher values give more refraction.
Normal used for shading.
Standard shader output.