The Glass Shader is similar to the Water Shader, but it simplifies the UI by removing some water-specific features and it includes extra settings which are often needed for objects made of glass or other materials with a glass-like appearance.
The Glass Shader can render subsurface scattering (SSS) which makes it useful for a wide range of materials that have this effect. Ice, smoky glass, ceramic and skin are examples of materials which have SSS. When using SSS, the results are best when rendering with Path Tracing.
See Also: Subsurface Scattering Guide